I had a few villagers born with the name "?". I was a bit confused at first, but I just figured it is a _bug_. I didn't realize it was a real issue until I got an island event where it said something like:

Your villager ? was hit on the head...

I was like, shouldn't there be a name in there, that's strange. Then my brain clicked on (becuase I have been a drone searching for collectables!) and realized it must be a glitch!