Are you using the KWeather plugin or Weather Report? In Plasma 6 I’ve had to use Weather Report, which uses the (I believe deprecated) DataEngine plugins. I’ve noticed in particular that the NOAA plugin will occasionally crash, leading to any results using their API to disappear.
KWeather uses KWeatherCore, which so far has been more accurate and stable for me. The “kweather_1x4” widget isn’t compatible with Plasma 6 though, so for now I’ve only been test with the application itself.
I wonder if it’d be worthwhile to add KWeatherCore functionality to Weather Report, for consistency’s sake. I don’t think there’s a direct migration path from the user perspective, though…