So, after the unresolved issues of not being able to adjust the volume of paired speakers by voice commands or routines, speakers unpairing at random, etc., starting this new year in line with the Google tradition of unleashing bugs instead of fixes, the following issue has risen:
When trying to cast Spotify or YouTube Music to paired speakers you now get the message: "This device (the paired speakers) doesn't support casting from this app (Spotify or YouTube Music)." This is if you are lucky enough that those paired speakers even show up in your Spotify or YouTube Music app.
No problem with unpaired speakers.
To Google, I have this to say: "Sorry. Something went wrong. If you are ready, give it another try."