The new Christmas item for the tail has problems in being fully displayed while walking/jumping.

Yes some of the items have that problem.
A mouse hitbox is 30p squared but it has a bigger visual image limit.
Anything exceeding that limit will be chopped off and you can see it with some items.
If you also have the big shaman skill, you can notice it more often.

I had a feeling it's not only this item. I guess that it was right.
Yeah, I forgot about that skill, since I'm not using it.

MMhmm watch your moth