I don't know if it's a bug or not. Do they need time to spawn? Do they not spawn in winter? It was winter in my world when Hallowed Nights started. I'm into spring now.
I know the event is on, I can build the event specific items and my costume is available to wear.
But I've gone over my entire world now, and not a single trinket to be found.
I was on DST when I saw on the forums that the Hallowed Nights event started, I relogged into the game and saw the event specific built items (like Treat Bag) were up and my costume. But I've yet to see a single trinket to turn into Pig King for candy.