After wearing or being near someone wearing the High Council Robes (both the new ones and Swifts special vintage ones) only the outer layer of your hand will render. All other people near you will also lose their inner hand layers (at least from your perspective), which will also result in items that they are holding sticking out through their stomach.

NOTE: It seems this doesnt happen to your own hand if you use the alex variant.

How to reproduce:

1. Play with Regeneration version 4.0.8 on 1.14.4
2. Load into a world
3. Equip High Council Robes or have someone near you equip them
4. Observe

Screenshots:
https://imgur.com/a/VRTIA17

Might just rip poor Swifts armor out and re-do the whole animation system for 1.14.

Added note:
Happens with the guard outfits too. (also with alex skin, so maybe I was wrong about that part)

Do you need any more info for this?

I would ask you for more information if I did