A question was asked on Site A, where it collected answers before being migrated to Site B. On Site B it collected more answers and lived happily for a time. More than two years later, it was closed.
Because it had been migrated originally, the closure was treated as a rejected migration. This meant that answers that were migrated from Site A were locked and deleted on Site B's copy of the question, and the migrated question became eligible for automatic deletion as a rejected migration. If the closure happens soon after the migration, this is OK -- when a migration is rejected, the affected answers are undeleted on the originating site. Nothing is lost and no answers are duplicated.
However, if it's been more than 30 days since the question was migrated, this presents a problem. Migration stubs are deleted after 30 days, so there was no (undeleted) question on Site A to host those answers. The result is that they were deleted on both sites. Closing the question caused content to be lost.
Now you might argue that hey, it was closed, so how valuable could the question and its answers be (regardless of voting)? But we have logic that isn't "delete all the things", so regardless of how you feel about keeping closed questions, this behavior violated user expectations. When a non-migrated ("native") question is closed, nothing happens to its answers. This question had been on Site B long enough to be treated as a "native" question.
Also, site scopes and the definition of what constitutes a good, on-topic question do change over time. A question that was perfectly on-topic on the destination site at the time of migration may be considered off-topic later, and leaving those questions open presents the usual problem, but closing them doesn't necessarily mean that the migration shouldn't have taken place.
Migration is often confusing and counter-productive anyway, but so long as we're stuck with it, there should be a statute of limitations on migration rejections. If the receiving site doesn't close it within 30 days -- the time that, on the original site, we've decided it's not coming back -- then it should stop being treated as a migration. Site A is done with it; it should henceforth be treated the same way as if it had been asked on Site B in the first place. Specifically, if it's closed it's just...closed. Not sent back. Not left around with deleted/locked answers. Not annotated as a rejected migration. Just closed.
For reference: the case study that prompted this request.
Also, another example: before meta sites existed, it was common for people to post meta questions on Stack Overflow itself. Then, many of these were migrated to then-MSO (now MSE). Later on, many of these were closed as pertaining to only a specific site in the SE network, which has caused the old content to be deleted. Even though rejected migrations on meta sites do not cause answers to be deleted, the migrated question is still eligible for automatic deletion as a rejected migration, regardless of score. Here's an example (screenshot from before the migration history was cleared) that had +31 score at the time of deletion.