Initial suspicion: Apps I had explicitly killed (equivalent to Force Stop) would start running. Most of these apps had no background services (or any reason to run in the background) and no notifications to show either. But they did have one thing in common: the vendor.

Further suspicion: Apps remain killed, for long periods of time, if I don't start any of them.

Quick test: Kill all apps. Start them one by one. Check if other apps are now running.

Confirmation: Pull APKs from device; RE their code for IPC.