I am posting this because I have wasted a few days trying to resolve this issue, and past work arounds are incorrect or tedious.
Cause: Changing a speaker setting (such as wifi ssid/password) requires a stereo pair to be split first, make the changes and recreate the speaker group.
Afterword google will only attempt to stream to the old ghost speaker group (and fail). Renaming only makes things worse.
Problem: The google app is not removing the speaker group from devices in google assistant after splitting the group. So it tries to stream to the "ghost" of the old group. There will be multiple speaker groups with the same name.
Solution: There are previous threads on this...as the issue is not new. The Bug has not been fixed.
One requires the install of a very old HOME APP that makes it easy to see and delete the "Ghost Speaker Group" , this works but is tedious.
There is a way in the current app (well hidden), but the menu options listed in a previous post are no longer accurate.
As of today on the newest app on android....
goto Home App
goto google assistant
goto "Manage all assistant settings"
There will a list....I found all the "ghosts" listed there. You can tell a "real" device from a "ghost" because clicking on it will show "some information unavailable". You can delete the "ghosts" from there.