Not sure where to raise this. In a discussion with Ignatz yesterday I found out
that one is not supposed to use snapper rollback in a transactional
installation.

snapper rollback would create a new snapshot of the btrfs filesystem and set
that one as new default. Since snapper does not know about the overlay on /etc
it does not modify /etc/fstab either. So the new snapshot would reuse the
overlay fs of the current snapshot.

In contrast transactional-update rollback only sets the specified number as new
default. Ie doesn't create a clone. So that kind of rollback will just boot the
old snapshot and continue on the overlayfs of that snapshot.

That kind of behavior is quite surprising. Obvious solutions would be

1. have snapper recognize transactional mode and make rollback behave
differently
2. make snapper rollback a noop with message, similar to how zypper refuses
when in transactional mode.