Using Homepod(s) as AppleTV speakers has changed with tvOS 14.2
There is no longer an Audio Output setting in tvOS 14.2 under Video & Audio to connect Homepod(s) speaker(s) to the AppleTV. One has to use the Control Center on the Remote by pressing and holding the TV button until the Control Center appears on screen and selecting the AirPlay icon. See <https://support.apple.com/en-us/HT202809#appletv>
It took me a long time to find this method after updating my AppleTV HD but it does appear that tvOS 14.2 fixes the problem of losing the Homepod speaker setting between uses of AppleTV.