Hi! OP here. In my case Time Machine stopped doing backups to the server, period, and would keep silently refusing to do so. I encourage you to check backups are happening.

I would try to do some restores of random files. Kind of a "canary in the coal mine" test. If you have problems with restoring some files or folders, then you'll have a problem with doing larger restores.

I agree - I am running my own Samba server and I don't think I'm affected, but it isn't really clear to me how to double check or why Apple's new default broke things in the first place