Guardians, Elder Guardians and Armor are rendered wrong

I found an underwater monument and were preparing to go inside (in creative). I enchanted some armor and a sword equipped it, but it didn't show up correctly in the preview. I didn't care so much about it until the armor disappeared and became into those strange shapes you see in the picture. As i went to explore the underwater monument I discovered those strange shapes that was supposed to be guardians. I turned off the 3d block model resource pack I used, to see if that was the problem but the shapes still was there.

What I expected to happen was...:
The armor and the guardians rendered properly

What actually happened was...:
Instead, it dropped an apple.

Steps to Reproduce:
1. Find/spawn a guardian or equip armor
2. Look.
3. Notice that the armor and the guardians don't render correctly