This morning I noticed I had received two separate instances of +1 reputation on answers (one, two), both of which were labeled "undownvoted" in the reputation tab of my profile.
As I hadn't removed the downvote myself, this must've happened in some other way. Why were these two downvotes reverted?
As noted by Mage Xy in a comment, this was due to the serial voting script, though it's not just for downvoting. There's an explanation of it on Meta Stack Exchange: What is serial voting and how does it affect me?
While we don't reveal all of the triggers, it seems that you were caught by them. I'm not sure that the invalidation was necessarily correct in this case but you fell into a somewhat unusual situation. It looks like that user posted several low quality answers in an hour which I'm guessing you saw either in review or on the home page all at the same time and voted on.
The upside is that there's no punitive result of this invalidation - it's not even marked in a way the Arqade mods can see and I even missed it myself initially because of how it's obscured.
While I can't un-invalidate these votes, you can actually re-cast them if you like if you still feel that the answers deserve the votes. From the MSE post:
Votes reversed by the detection script can be re-cast by the user at a later time, so long as the user does not again engage in serial voting which causes them to be reversed (yet again).
