I have a brand new Chromecast with Google TV and I have it set up as part of a speaker group with a Nest Mini. Whenever I go to cast to the group, the TV's Now Playing screen is some strange looking Spotify UI that I've never seen before. It looks awful and is very low resolution.
When I cast to the TV alone, outside of the speaker group, the normal Spotify Now Playing screen comes up. Looks great.
I spoke with Google support for two hours this morning and they insist this is a Spotify problem and not a Chromecast issue. They say that they have no control over what Spotify chooses to display when casting to a TV, when it's in a group and when it's not in a group.
See photos below. I have done a full reset of my Chromecast and a full CLEAN install of the Spotify app on my phone.