My sister's pc's minecraft was stuck on 1.6.1, but the realm backup was saved when she accessed it using her phone, which had updated to 1.6.2 unbeknownst to me, so when I tried to restore the backup (and of course it failed), it deleted the backup without warning.

It's probably coded based on the assumption that a backup restoration failure is because of a corrupted file, therefore deleting it is probably best, but it is not ok in this case because the save was perfectly fine if minecraft would only update.