If you write manual migrations, then it's often worth it to write the reverse function (in your case, deleting that group) so that you avoid this situation.
It’s not the back and forth, it’s that a migration in b that requires the post migrate step from a will fail if both migrations are run in one shot, (say, because you’re bootstrapping a new dev or test db) but it will work when done incrementally (such as during development or deployment).