@killclique
I may have to eat my words here. After doing the test as described in my previous post, I just stopped Waydroid, stuck the phone in my pocket and got out.
Several hours later, I wanted to use my phone and had an experience that looks like what you said, but a bit different: I could vaguely see the lock screen but as seen behind a very dark screen, and I did not succeed in unlock the screen. After fiddling for about a minute trying to bring the phone to life, it did go darker still, I could not even see anything. I restarted the phone and it worked as normal, so it was not a case of battery gone to nothing.
I did not relate immediately this problem to your post because it was slightly different - I had Waydroid stopped. But now that I think of it, maybe it was. If it's true, starting Waydroid and letting the phone go to sleep could have unexpected, long delayed consequences. If I had not seen it before, maybe it's because I almost never use Waydroid.
