I have a Mu-so Qb 2nd Generation designated as the default speaker for the Nest device in my kitchen. Using a “hey Google” voice command, it works if Youtube Music is set as streaming service, but if Spotify is, it first says “playing on Spotify on Kitchen Speaker”, followed by “something went wrong”. In every other room, with Nest speakers, Spotify works.
Playing Spotify on the Mu-so using Spotify Connect from a phone works.
Can’t help with the specifics, but if you hunt around this forum, you’ll discover the endless problems I had trying to get Google Home/Google Assistant to work reliably with my Naim products. Bottom line (as of about a year ago) - it’s unstable. I gave up and switched to Alexa.