I'm fairly sure this is by-design. Your reputation change (in the tab) gets updated upon a downvote, to sum to -2 for it. However, when events get reversed, the system doesn't also revert the change in the tab (it's probably a hassle to keep track of what events make up the reputation change you see there, since it resets whenever you dismiss it). This is similar to what happened in another question.
When a downvote gets reversed, you're not gaining 2 reputation, the -2 you received before is being reversed. The actual undownvote event in your reputation history is just a nice notice so you know why your reputation went back up by 2. Since it's reversing a previous event, it doesn't update the reputation change in the tab. So the -2 is from the downvote only, the undownvote is ignored.
As for whether or not it's a bug, I don't see any reason for the developers to spend much time on this. If it's not a simple fix for them, I'd say let it go. It does reset as soon as you look at your history, after all.