3D Armor - It just randomly doesn't work!

3D Armor is a nightmare!

I have been trying all day to create some armor with a 3D model in 1.16.5. It isn't complicated, just a normal skin, with a layer (making 3D needed) yet it has been such a pain. 

After much trying, the boots and chestplate work perfectly, yet for no apparent reason, (even if I make them in the exact same way), the helmet and leggings have extra random blobs, in random places away from the model. Like for the helmet, it renders perfectly well, yet there is an extra plane on top, maybe a pixel or two up, that is just a squashed version of the front and top of the main helmet.

I have done everything that I can find online about similar issues, yet none seem to work!

Any help is greatly appreciated! I'm not entirely sure what sort of stuff would be useful, so just ask for whatever.

Many thanks.