I’m guessing that you are using MQTTSubscribe…
It depends on how the sensor is 'reporting' rain. WeeWX expects the observation rain to be a delta value. If the sensor is 'reporting' a total value, it needs to be converted to a delta value. While MQTTSubscribe can do the conversion (see,
https://github.com/bellrichm/WeeWX-MQTTSubscribe/wiki/Configuring#contains_total), starting in 4.2/4.3 this functionality is built into WeeWX. Some information on configuring WeeWX can be found here,
https://groups.google.com/g/weewx-user/c/FYHyoM9AfGw. Since calculating a delta is now built into WeeWX, this functionality will eventually be removed from MQTTSubscribe.
It also might be a unit conversion 'error'.
Based on the information provided, those would be my first guesses.
rich