I now have further proof that Android 9's fancy Battery Management is shit:
The compass on my new (flagship) smartphone uses the air pressure sensor and the air pressure at sea level to display your current elevation. The air pressure at sea level is fetched from the weather app. And of course, the weather app has its battery policies set to auto-managed.
Can you guess what happened? "Weather app could not be found" and an elevation of -17m
Because no one thought the battery manager could decide that the weather app should not be launched by other apps. Has no one thought this through? Good thing I figured out what was happening pretty quickly.