Either this has been fixed or something fishy is going on, but I was able to finish this one by renaming a butterfly. If I see more reports of this I'll investigate further. Thanks Skywise!
I'm betting it was just a delayed reaction to the quest being solved. The quest was correctly finished when the butterfly was renamed, but the pop-up didn't appear. Then when Skywise tried naming the batterfly, the game finally caught up and acknowledged the quest completion. The timing was likely a wild coincidence.