With 14.2 Apple introduced Home Theater Audio. Unfortunately, the HomePod Mini is not able to support this form of output, even when the Mini's are set up as stereo pairs.
The reason you are able to view the Mini(s) in the AppleTV AirPlay menu is because things like music and podcasts can still airplay audio to any AirPlay 2 devices because it's not in HDR/DolbyDS, which I assume is what Apple's "Home Theater Audio" is referring to. This is also the reason you can see the Mini(s) in the AirPlay menu, but cannot select a Mini as a default audio output source in tvOS settings.
I suggest we use the feedback site requesting Apple engineering consider an update to tvOS to support automatic audio-output formatting so it may adjust based off the selected output source. Personally, I don't need Dolby or HDR audio in my bedroom, but the Mini's are certainly an upgrade from the TV's internal speakers.
I purchased 2 Mini's for the purpose of having AirPlay for my bedroom Apple TV. I assumed it was a small glitch that would work itself out with a software update until I happened upon this forum, which led me to research and find that it is simply not possible.
Giving Apple the benefit of the doubt to resolve this "issue" with an update has consequently put me out of the return/exchange window. Had I known this was not a supported feature, I'd have opted to purchase just one HomePod for bedroom audio and left the 2 Mini's on the shelf.
Additionally, the cost of my 2 Mini's (intended for Stereo Pairs for my TV output) falls JUST shy of AirPod's Pro, which are supported for Home Theater output from tvOS even though they're primarily marketed as an on-the-go audio solution. Meanwhile, Apple is marketing a HOME speaker (the mini) that is manipulated and managed in the HOME app and it lacks the ability to be a source of audio for AppleTV located in HOMES while my AirPods Pro have total TV AirPlay functionality.
I literally own every single Apple product released since 2010. I get the latest anything as soon as it comes out. I love the all-Apple-product life because it's one of the most seamless, easy, secure ecosystems of hardware, software, and services in the world. The innovation Apple turns out in their products/services is exciting, inspiring, and doesn't disappoint.
But this time.. for the first time as an Apple user, I am totally let down.