Correct me if I'm wrong but it seems that ticks are uploaded from your local mountain project app to the website either when you next open the app with internet connection or at some other later point.
I've noticed that if you tick climbs near the transition of the new year, say Dec 31st 2024, the tick will be uploaded as Dec 31st 2025. This seems to no longer be a problem after Jan 1st, and only a problem for ticks made a few days prior to the new year.
You can see some examples by checking out the ticks of some popular climbs that are often climbed around the new year, you'll find a handful of ticks from the future.
It's a bug that goes back to at least 2019, and I think it has been pretty low priority since, as you pointed out, it only occurs in the last few days of the calendar year.
There's another recent example demonstrated in the thread here:
Thanks, Brayden, and big thanks to Annie. We've got this bug written up, just need to dig in and figure out how to resolve. You should be able to edit the tick year on web as a workaround? But I haven't fully tested that recently.