The bug
The top texture of the librarian villager and zombie villager professions is rotated 180 degrees. Looking in the assets shows this to be a problem with the texture itself and not any coding error.
The blue arrows denote where the white strap should be if the texture were correct.
It's possible that this applies to other villager related textures as well, but it only seems noticeable to me on the librarian textures
Edit for clarification: I found this while making a resource pack which is why the librarian profession texture is overlayed over the villager texture.
Affects 1.15 Pre-3