Hello,
I think I have a similar issue with a MacBook Pro 14'' 2021 with Monterey 12.0 and AirPods Max (can't really say witch firmware's on it)
When looking for spatial audio on audio control center, its greyed out just like you. In Music preferences, although, I can set Dolby Atmos preferences, unlike you. When I tried to play Dolby Atmos content from music app (streaming) the spatial audio toggle from control center is still greyed out but the mini player

If I disable the automatic or always on option in music preferences the dolby symbol disappears and the sound changes (becomes less engaging).
Despite this, I still get the spatial audio greyed out in control center.
The same tracks played through my iPhone get the Spatial sound option in control center. None of the tracks are downloaded so that it will only work with downloaded tracks must not be true.
I'm updating to Monterey 12.0.1as I type, lets see if it changes anything...
Just my two cents to the discussion as I bugs the **** out of me too...