Take in mind that upgrading the shell you're also using libmutter-10 instead of libmutter-9, so the problem may be anywhere in both (more likely in mutter).
So the only way to find out what may have changed this is using git bisect over the various mutter versions (might be annoying, because you need to keep shell in sync with that).
If the unconfined snap isn't affected, could be that mutter is now using something that is blocked by the confinement?
Take in mind that upgrading the shell you're also using libmutter-10 instead of libmutter-9, so the problem may be anywhere in both (more likely in mutter).
So the only way to find out what may have changed this is using git bisect over the various mutter versions (might be annoying, because you need to keep shell in sync with that).
If the unconfined snap isn't affected, could be that mutter is now using something that is blocked by the confinement?