Chrisback, You’re not crazy!! My AppleTV 4K does the exact same thing. It will even start playing on its own in the middle of the night. I briefly played with it trying to figure out the problem(force quitting apps, etc) before I gave up and did a full reset with latest firmware re-install. That did not fix the issue.
So I dug a little deeper and discovered it will only start playing when the sound output is set to an Airplay speaker. My setup has the Apple TV plugged into my TV, but the sound is always set to my Bang & Olufsen A9 via Airplay. If I sleep the AppleTV while its still connected then when I wake it up music will start playing. If I set the output back to the TV before sleeping the ATV it will NOT auto play.
I have also found that when it starts playing on its own the ATV is still somewhat sleeping. When the music starts coming from my A9 I can turn on the TV but the ATV(My only input) is still sleeping. When I click the remote it wakes up and its video will start a few seconds later and I can go to the Music app to see that it is indeed playing from my Apple Music Library.
At this point it appears to be a bug but I have been unsuccessful finding anyone else with the same problem. I cannot say that the problem started with TVOS 12 but it started happening within the last few months and I’ve had AppleTV’s since the original.
JT