Apple TV 4K occasionally starting up with wrong audio output

I've solved my problem. The issue was that I had a homekit automation set up with one of it's actions being to "resume audio" on one of the sonos (actually ikea symfonisk) speakers. I guess the meaning of "resume audio" is not so straightforward - I had been assuming it was equivalent to pressing the play button on the speaker or in the sonos app, in which case it resumes whatever was playing, whether that be from the sonos library, one of the sonos streaming services or airplay. Instead the homekit "resume audio" seems only to resume airplay streams, and if there is not a paused airplay stream to that speaker then it for some reason changes the apple TV output to airplay to that sonos. Presumably this is related to the fact that the apple TV is my home hub?

Anyway, the intermittent triggering of this automation, combined with the dependency on what was previously being played on the speaker in question, made this tricky to track down. But hopefully I included enough keywords in this answer for anyone in the future having similar problems to find this solution via google.

Thanks for you reply.

When this problem happens I've checked the TV was still set to output to the Sonos.

Every time this problem came up I've (temporarily) fixed it by using the control center to set Apple TV output back to "TV speakers" instead of the Sonos airplay that was somehow selected each time - the issue is that it doesn't always stay that way forever, and for some reason eventually switches un-commanded to a Sonos speaker via airplay rather than staying on "TV speakers" as intended.

I'm not sure how often is "once in a while" since I wasn't really tracking it for the first 3 or 4 times, given that it was infrequent I didn't notice the pattern. After I started paying closer attention, I see it happened once today and once yesterday.

I've selected Airplay speakers a couple of times just to play around with that feature when I bought the Apple TV a few weeks ago, but not used it at all since then, because I don't have any actual use case for airplay from the TV. I'm also certain no one else in the house ever uses airplay from the TV on purpose. Although its possible that something they are doing something different from me can trigger the issue - eg, while I always use the Siri remote, my wife sometimes uses the TV remote to turn off the system or change the volume, or uses the iPhone remote app, the kids let it time out and go to sleep, the kids use their bedroom Sonos to listen to their streaming music, sometimes I listen with AirPods Pro... I tried everything I could think of but couldn't replicate the issue other than just by waiting.

I did some google searches also before posting here, and agree that this doesn't seem to be a common problem that anyone else is experiencing.

You're right that I just "turn off" with the power button on the Siri remote, not rebooting the hardware. If it happens again I'll try a reboot and I suspect you're correct that after the reboot it will select the TV audio.

I just updated TvOS to 15.4.1 and rebooted, and so far no recurrence, but its only been a few hours and of course with this kind of intermittent problems it's not clear how long to wait until deciding the problem has gone away.