It sounds like your WiFi and Gateway are working OK.
Unlike other sensors, that send readings, the BME280 and BMP280 sensors are more complex and send 4 back-to-back special messages to the Gateway that it combines to extract the Temperature, Humidity, and Pressure values. If there is any glitch by the receiver of the 4 messages all is lost for that update. It sounds like the receiver is misinterpreting some parts of the 4 messages and generating a bad result.
We have been working on a redesign of the BME280 sensor but that has been causing other issues.
One solution that seems to help is moving the sensor closer to the Gateway to reduce the errors.