I have the same issue. On my back deck, I have TV connected to a ATV 4 (A1625). With tvOS14, I had the audio default go to an Airplay Yamaha receiver that drove speakers on the deck. The TV speakers are off.
It was perfect - I turned on the TV with the Apple remote and everything just worked.
With the update, now I don’t have the option to select the default audio output to my Airplay receiver. I can select the Airplay receiver as a temporary option but it doesn’t appear as an option on the default audio output. On top of that, everything is out of sync.
The wireless audio sync does not work because it seems to want the audio signal to default the sound to the TV speakers and not to the temporary audio selection so I can’t adjust the audio sync.