I thought that also, even with the suspicious timing, until I read this thread - others seem to having the exact same issue after upgrading...
Just ran a btrfs scrub (back at 6.11.5), with no errors.
Can the new 6.12.6 kernel/btrfs version have improved checks, that do not exist in 6.11.5?
Anyway, if everything is ok for a few days, I'll try the upgrade again (disabling bridging).
I had macvlan issues in a previous upgrade (6.12.0), but not like these - this time everything was working wonderfully, up until the corruption errors
Thanks for taking the time to look into this!