Custom item's swing animation does not match vanilla swing animation in first person

Custom items uses a totally different swing animation than what vanilla items use in first person. This happened since 1.16.100. (I cannot select older versions for some reason.)

I have attached a test pack, 2 videos showing how it looks like, as well as 2 images showing the first frame of the swing animation. Attachables seem to also have a strange swing animation on items.