In previous versions that I've used (Forge 1.16), the Hand Model that's rendered in first person always renders from the Right/Left Arm Model, however in this version the Hand doesn't match up with the Arm Model and the texture breaks completely if you have a texture sheet bigger or smaller than the Vanilla default of 64x64

What it should look like:

What it actually looks like:

Keep in mind both the vanilla and custom Right Arm Models look the same.

Edit: Another thing I should mention is that I used the convert Vanilla parts tool after writing this and the same thing happened again.

• Slight GUI Modifications
• Animatica
• Architectury API
• ARRP
• BetterF3
• CIT Resewn
• Clear Despawn
• Clear Skies
• Cloth API
• Cloth Config API
• Colormatic
• Continuity
• Cursor Mod
• Custom Entity Models
• Custom Splash Screen
• Entity Texture Features
• Fabric API
• Indium
• Iris Shaders
• LambdaBetterGrass
• LambDynamicLights
• Lithium
• Mod Menu
• Not Enough Crashes
• Ok Zoomer
• Phosphor
• Replay Mod
• Sodium

It may be a incompatibility with another mod. Try it without Entity Texture Features, Custom Entity Models and CIT Resewn to see if it works.

What other mods do you have installed?

Yep, it worked

I tested it some more and it seems to be an issue with Entity Texture Features specifically

I tested it some more and it seems to be an issue with Entity Texture Features specifically

Because Entity Texture Features has a thing where you can add additional parts to your player only in the skin file.