Android, lay cell down no notification tone

I've met phones where you can set them to mute by placing them face down, but never one that does it face up. It might be worth checking through your system settings, especially sound-related but also anything related to customisation, notifications, gestures, etc, just to make sure that there isn't an option in there that has been set to a strange default ("default" because a factory reset will set all settings to their defaults). Look for any option that might mute based on orientation: it could be that a problem with the senor has changed how it behaves, so I'd turn off anything that might be orientation-dependent.

And when you say "no new apps installed" do you mean "I didn't restore any apps after the factory reset" or "I've not installed any new apps around the time this started"? If the first then fine. If however you did restore the apps you previously had installed you may have restored the cause of the problem. Not having installed new apps doesn't exclude the possibility that an update to an old one caused this. Maybe try running in "safe mode" for a bit: if the problem is still there then it's part of the system or one of the apps pre-installed by the manufacturer. If the problem goes away in safe mode then it's one of the apps you have installed.