I can't reproduce this. This is a situation that I do quite a lot in the bugs forum, but the old prefix is always shown as "-". This should only happen if there is no "old" key stored in the log.
Was this generated by a standard XF action or an add-on?
Select a thread with no prefix
Move threads...
Go
Destination forum: Select forum
Select Apply prefix to selected threads
Select a prefix
Do not leave a redirect
Move Treads