This is my logs (I grepped for outTemp). I wonder if the issue is coming from the fact that WMR200 has an outTemp but it is faulty so I have outTempFault set to 1.
Nov 11 16:39:42 vegan weewx[30478] INFO weewx.drivers.wmr200: sensor map is {'outTempBatteryStatus': 'battery_status_out', 'outHumidity': 'humidity_1', 'rainRate': 'rain_rate', 'heatindex4': 'heatindex_5', 'rainTotal': 'rain_total', 'rainBatteryStatus': 'battery_status_rain', 'heatindex6': 'heatindex_7', 'extraTemp5': 'temperature_6', 'uvFault': 'uv_fault', 'extraTemp2': 'temperature_3', 'heatindex': 'heatindex_1', 'extraTemp3': 'temperature_4', 'rain24': 'rain_24', 'inHeatindex': 'heatindex_0', 'inTemp': 'temperature_0', 'extraTemp1': 'temperature_2', 'hourRain': 'rain_hour', 'windFault': 'wind_fault', 'windchill': 'windchill', 'clockUnsynchronized': 'clock_unsynchronized', 'extraTemp7': 'temperature_8', 'rain': 'rain', 'forecastIcon': 'forecast_icon', 'pressure': 'pressure', 'extraHumid4': 'humidity_5', 'extraHumid5': 'humidity_6', 'extraHumid2': 'humidity_3', 'extraHumid3': 'humidity_4', 'extraHumid1': 'humidity_2', 'extraTemp6': 'temperature_7', 'heatindex5': 'heatindex_6', 'extraTemp4': 'temperature_5', 'heatindex7': 'heatindex_8', 'altimeter': 'altimeter', 'heatindex1': 'heatindex_2', 'heatindex2': 'heatindex_3', 'heatindex3': 'heatindex_4', 'extraHumid6': 'humidity_7', 'extraHumid7': 'humidity_8', 'windBatteryStatus': 'battery_status_wind', 'UV': 'uv', 'rainFault': 'rain_fault', 'windDir': 'wind_dir', 'outTemp': 'temperature_1', 'windSpeed': 'wind_speed', 'inHumidity': 'humidity_0', 'outTempFault': 'out_fault', 'windGust': 'wind_gust', 'uvBatteryStatus': 'battery_status_uv'}
Nov 11 16:41:46 vegan weewx[30694] INFO weewx.drivers.wmr200: sensor map is {'outTempBatteryStatus': 'battery_status_out', 'outHumidity': 'humidity_1', 'rainRate': 'rain_rate', 'heatindex4': 'heatindex_5', 'rainTotal': 'rain_total', 'rainBatteryStatus': 'battery_status_rain', 'heatindex6': 'heatindex_7', 'extraTemp5': 'temperature_6', 'uvFault': 'uv_fault', 'extraTemp2': 'temperature_3', 'heatindex': 'heatindex_1', 'extraTemp3': 'temperature_4', 'rain24': 'rain_24', 'inHeatindex': 'heatindex_0', 'inTemp': 'temperature_0', 'extraTemp1': 'temperature_2', 'hourRain': 'rain_hour', 'windFault': 'wind_fault', 'windchill': 'windchill', 'clockUnsynchronized': 'clock_unsynchronized', 'extraTemp7': 'temperature_8', 'rain': 'rain', 'forecastIcon': 'forecast_icon', 'pressure': 'pressure', 'extraHumid4': 'humidity_5', 'extraHumid5': 'humidity_6', 'extraHumid2': 'humidity_3', 'extraHumid3': 'humidity_4', 'extraHumid1': 'humidity_2', 'extraTemp6': 'temperature_7', 'heatindex5': 'heatindex_6', 'extraTemp4': 'temperature_5', 'heatindex7': 'heatindex_8', 'altimeter': 'altimeter', 'heatindex1': 'heatindex_2', 'heatindex2': 'heatindex_3', 'heatindex3': 'heatindex_4', 'extraHumid6': 'humidity_7', 'extraHumid7': 'humidity_8', 'windBatteryStatus': 'battery_status_wind', 'UV': 'uv', 'rainFault': 'rain_fault', 'windDir': 'wind_dir', 'outTemp': 'temperature_1', 'windSpeed': 'wind_speed', 'inHumidity': 'humidity_0', 'outTempFault': 'out_fault', 'windGust': 'wind_gust', 'uvBatteryStatus': 'battery_status_uv'}
Nov 11 16:46:29 vegan weewx[30993] INFO weewx.drivers.wmr200: sensor map is {'outTempBatteryStatus': 'battery_status_out', 'outHumidity': 'humidity_1', 'rainRate': 'rain_rate', 'heatindex4': 'heatindex_5', 'rainTotal': 'rain_total', 'rainBatteryStatus': 'battery_status_rain', 'heatindex6': 'heatindex_7', 'extraTemp5': 'temperature_6', 'uvFault': 'uv_fault', 'extraTemp2': 'temperature_3', 'heatindex': 'heatindex_1', 'extraTemp3': 'temperature_4', 'rain24': 'rain_24', 'inHeatindex': 'heatindex_0', 'inTemp': 'temperature_0', 'extraTemp1': 'temperature_2', 'hourRain': 'rain_hour', 'windFault': 'wind_fault', 'windchill': 'windchill', 'clockUnsynchronized': 'clock_unsynchronized', 'extraTemp7': 'temperature_8', 'rain': 'rain', 'forecastIcon': 'forecast_icon', 'pressure': 'pressure', 'extraHumid4': 'humidity_5', 'extraHumid5': 'humidity_6', 'extraHumid2': 'humidity_3', 'extraHumid3': 'humidity_4', 'extraHumid1': 'humidity_2', 'extraTemp6': 'temperature_7', 'heatindex5': 'heatindex_6', 'extraTemp4': 'temperature_5', 'heatindex7': 'heatindex_8', 'altimeter': 'altimeter', 'heatindex1': 'heatindex_2', 'heatindex2': 'heatindex_3', 'heatindex3': 'heatindex_4', 'extraHumid6': 'humidity_7', 'extraHumid7': 'humidity_8', 'windBatteryStatus': 'battery_status_wind', 'UV': 'uv', 'rainFault': 'rain_fault', 'windDir': 'wind_dir', 'outTemp': 'temperature_1', 'windSpeed': 'wind_speed', 'inHumidity': 'humidity_0', 'outTempFault': 'out_fault', 'windGust': 'wind_gust', 'uvBatteryStatus': 'battery_status_uv'}
Nov 11 16:46:30 vegan weewx[30993] DEBUG user.MQTTSubscribe: (Service) service_dict is {u'enable': u'true', u'host': u'192.168.0.9', u'port': u'1883', u'keepalive': u'60', u'username': u'None', u'password': u'None', u'binding': u'loop', u'message_callback': {u'type': u'individual'}, u'topics': {u'unit_system': u'METRIC', u'temperature/jardin': {u'name': u'outTemp', u'ignore': u'False', u'contains_total': u'False', u'conversion_type': u'float', u'units': u'degree_C'}}}
Nov 11 16:46:30 vegan weewx[30993] DEBUG user.MQTTSubscribe: (Service) service_dict is {u'enable': u'true', u'host': u'192.168.0.9', u'port': u'1883', u'keepalive': u'60', u'username': u'None', u'password': u'None', u'binding': u'loop', u'message_callback': {u'type': u'individual'}, u'topics': {u'unit_system': u'METRIC', u'temperature/jardin': {u'name': u'outTemp', u'ignore': u'False', u'contains_total': u'False', u'conversion_type': u'float', u'units': u'degree_C'}}}
Nov 11 16:46:30 vegan weewx[30993] DEBUG user.MQTTSubscribe: (Service) TopicManager config is {u'unit_system': u'METRIC', 'overlap': 0.0, u'temperature/jardin': {u'name': u'outTemp', u'ignore': u'False', u'contains_total': u'False', u'conversion_type': u'float', u'units': u'degree_C'}}
Nov 11 16:49:36 vegan weewx[30993] DEBUG user.MQTTSubscribe: (Service) data-> final packet is 2020-11-11 16:47:15 CET (1605109635): clockUnsynchronized: 1, dateTime: 1605109635, outTempBatteryStatus: 0, outTempFault: 1, rainBatteryStatus: 0, rainFault: 0, usUnits: 16, uvBatteryStatus: 0, uvFault: 0, windBatteryStatus: 1, windFault: 0
Nov 11 16:49:36 vegan weewx[30993] DEBUG user.MQTTSubscribe: (Service) data-> final packet is 2020-11-11 16:48:09 CET (1605109689): clockUnsynchronized: 1, dateTime: 1605109689, outTempBatteryStatus: 0, outTempFault: 1, rainBatteryStatus: 0, rainFault: 0, usUnits: 16, uvBatteryStatus: 0, uvFault: 0, windBatteryStatus: 1, windFault: 0
Nov 11 16:49:36 vegan weewx[30993] DEBUG user.MQTTSubscribe: (Service) data-> final packet is 2020-11-11 16:48:53 CET (1605109733): clockUnsynchronized: 1, dateTime: 1605109733, outTempBatteryStatus: 0, outTempFault: 1, rainBatteryStatus: 0, rainFault: 0, usUnits: 16, uvBatteryStatus: 0, uvFault: 0, windBatteryStatus: 1, windFault: 0
Nov 11 16:49:36 vegan weewx[30993] DEBUG user.MQTTSubscribe: (Service) data-> final packet is 2020-11-11 16:49:08 CET (1605109748): clockUnsynchronized: 1, dateTime: 1605109748, outTempBatteryStatus: 0, outTempFault: 1, rainBatteryStatus: 0, rainFault: 0, usUnits: 16, uvBatteryStatus: 0, uvFault: 0, windBatteryStatus: 1, windFault: 0
Nov 11 16:50:15 vegan weewx[30993] DEBUG user.MQTTSubscribe: (Service) data-> final packet is 2020-11-11 16:50:12 CET (1605109812): clockUnsynchronized: 1, dateTime: 1605109812, outTempBatteryStatus: 0, outTempFault: 1, rainBatteryStatus: 0, rainFault: 0, usUnits: 16, uvBatteryStatus: 0, uvFault: 0, windBatteryStatus: 1, windFault: 0
Nov 11 16:51:15 vegan weewx[30993] DEBUG user.MQTTSubscribe: (Service) data-> final packet is 2020-11-11 16:51:08 CET (1605109868): clockUnsynchronized: 1, dateTime: 1605109868, outTempBatteryStatus: 0, outTempFault: 1, rainBatteryStatus: 0, rainFault: 0, usUnits: 16, uvBatteryStatus: 0, uvFault: 0, windBatteryStatus: 1, windFault: 0
Nov 11 16:52:15 vegan weewx[30993] DEBUG user.MQTTSubscribe: (Service) data-> final packet is 2020-11-11 16:52:08 CET (1605109928): clockUnsynchronized: 1, dateTime: 1605109928, outTempBatteryStatus: 0, outTempFault: 1, rainBatteryStatus: 0, rainFault: 0, usUnits: 16, uvBatteryStatus: 0, uvFault: 0, windBatteryStatus: 1, windFault: 0
My MQTT topic is "temperature/jardin" and I directly put as message the temperature e.g 16.5.
This is how I configured MQTTSubscribe (as a service):
[MQTTSubscribeService]
# This section is for the MQTTSubscribe service.
# Turn the service on and off.
# Default is: true
# Only used by the service.
enable = true
# The MQTT server.
# Default is localhost.
host = 192.168.0.9
# The port to connect to.
# Default is 1883.
port = 1883
# Maximum period in seconds allowed between communications with the broker.
# Default is 60.
keepalive = 60
# username for broker authentication.
# Default is None.
username = None
# password for broker authentication.
# Default is None.
password = None
# The binding, loop or archive.
# Default is: loop
# Only used by the service.
binding = loop
# The message handler to use
[[message_callback]]
# The format of the MQTT payload.
# Currently support: individual, json, keyword
# Must be specified.
type = individual
# The topics to subscribe to.
[[topics]]
# Units for MQTT payloads without unit value.
# Valid values: US, METRIC, METRICWX
# Default is: US
unit_system = METRIC
[[[temperature/jardin]]]
name = outTemp
ignore = False
contains_total = False
conversion_type = float
units = degree_C