When you start the event on the TV, it gives you like 9 in-game quests. Have you completed them all?
You need to hit Claim on each of those in-game quests to actually complete them.
After completing all in-game xmas quests, the next time you interact with the TV channel it should run the event ending which unlocks the event's gallery entry as well as marking the Treasure Hunt objective done.
Feel free to join our Discord if you're still having issues so I can help you troubleshoot more.