Ok, I have submitted Feedback FB17432373 with an extremely simple Xcode project that reproduces the issue.

The built-in Weather app's graph of "Next Hour Precipitation" does not display any numbers (only "light"–"heavy"), so I could not directly compare to the precipitation intensity values returned by WeatherKit. But my own app has a similar "light"–"heavy" scale which used to match up well with the Weather app but at some point over the summer the bars shot up and now they only match when I divide the inputs from WeatherKit by 2.54.