I am having an issue as well. I had 2 speakers connected for over a year now.
One of them went offline in smartthings and not matter what I do, I can’t add it back.
It still works through alexa because alexa is also connected to the bose skill but not through smartthings.
I tried removing it, clear the cache and doing the discovery.
I also tried to remove it, re-attaching it to the soundtrack app (successfully) and then do the discovery.
The 2nd speaker is still connected to smartthings and working without an issue.
Any thoughts?