Happened to me with a santa hat from 2016. I believe it is a bug, apparently it didn't appear in my purchase logs or anything so the staff team mever returned :[[. But I mean try saying showtots or whatever the command is for showing the treats from halloween and if it works ask a staff to view your total count and could probably somehow find that you earned the hat. Or if you have any hat above it (like by wearing one of the hats you recieve from more knocks).