Well, we didn't know that it's intended behavior... https://appleinsider.com/articles/21/06/07/homepod-mini-can-be-set-as-default-speaker-for-apple-tv-4k-in-tvos-14

Brand new AppleTV 4K 2021 running tvOS 14.6 replacing our old AppleTV where all was good. With the new one it‘s even weirder as our stereo pair of Homepod minis can‘t be set as default although a pair of original Homepods in the same room can. Additionally, there’s more complication: The stock music app has seemingly less restrictions as the temporary option works with it and the Homepod minis, whereas others (the NextPVR app for example) output their audio stream to the TV speakers — although the temporary option has not been changed. Now going for the 14.7 18M5542c beta… [EDIT:] Still the same bugs…