A similliar thing happened to me the day before where my automation that plays radio on both my upstairs speakers (kitchen and livingroom) started responding with "I cannot find <upstairs>". The speaker group is called 'upstairs'. This automation has been in place for months and never had an issue. I verified i could transfer to the group manually, so i deleted the automation and recreated it to be exactly like it was.... and it started to work.
Fast forward 2 days later, and this morning i run the automation - and hear "I cant find <upstairs>". However, this time, when manually start music on 1 speaker, and manually "transfer to upstairs speaker group", it say, "Ok il play it on <device i am talking to>" and not the group. So i made a brand new speaker group using the google default "home group" name, and tried to transfer to it, and it did the same thing ... only referencing 1 speaker i was talking to. So i tried to say transfer to the <other speaker> not the group.... and it replied "ok ill play it on HOME GROUP" and then only played it on the other speaker and not the group. Also casting the the group also works for me fine as you noted.
Either something has broken speaker groups in the last week, or google is trying to dive me insane.......
.
.
