As a result of the ability to "reject" a migration (through certain types of closure on the target site, or deletion on the target site), answers will be undeleted on the source site when a migration is rejected.
However, answers that are not deleted before the question is migrated are undeleted as well.
Take this example on Stack Overflow (links 10K only):
However, take this answer, which was deleted before migration took place:
Which did not appear on the target site:
However, when the migration is rejected, the answer is undeleted.
Furthermore, if the question is deleted after the migration is rejected and then undeleted, the answer that was originally deleted gets undeleted (which is what I would expect when the migration is rejected).
So it seems that a check needs to be performed to make sure that answers that are deleted when migrated remain deleted when that migration is rejected.