There is no bug
You can never be sure with downvotes, even mods have somewhat limited information on this. But to get +4 rep like that you would have to have a post with two downvotes (and no upvotes) deleted. The system would give you back the 4 reputation taken by downvotes. Moderators deleting a post carries no penalties other than the potential change in reputation.
You indeed do have a post that meets those criteria, so I would say that this is what happened.