Fun fact: Android has "Doze" mode.

If a user leaves a device unplugged and stationary for a period of time, with the screen off, the device enters Doze mode. In Doze mode, Android suspends network access, ignores wake locks, defers alarms, doesn't let sync adapters, JobScheduler and WorkManager run.

That is why your notifications don't show up unless you use Google services, which are.. shockingly.. exempt from Doze.