Alright Fairly sure this is a bug but tracking triumphs for seasonal event has some weird interactions between the seal page and the event page.
If you track one on the event page and try to untrack it on the seal page it will not untrack
If you track it on the event page and claim it on the seal page it will remain tracked
If you track it on the seal page and try to untrack it in the event page it will stay tracked
Somewhere along the line you can track them on both pages making it hard to untrack it entirely