This might be another design issue or it may be as designed, but we'll see. Let's take a prefix from XenForo: Not a Bug. Mike moves a thread to Resolved Bug Reports and prefixes it Not a Bug. However, the reporting user later realizes they made a typo in the title and goes to fix it; after they fix it, the thread is without a prefix because the user group the user is a part of doesn't have access to any of the resolution prefixes and the prefix Not a Bug was therefore removed during the process of changing the thread title. Even if we don't have access to certain prefixes and can't change the prefix, whatever prefix is already there should remain in place.