Wearing a Netherite Backtank from Create prevents the player's arm from being rendered in first person. Not sure if this is an issue with this mod or create, but I thought I'd start here.

Baseline of what things look like without wearing anything.

Me wearing a Netherite Backtank without 3d Skin Layers

And finally, me wearing a Netherite Backtank with 3d Skin Layers.

Read about some of the other issues here. Compatibility mode does appear to solve this problem.

Compatibility mode is the default by now (the non compatibility mode is gone), so should be fine.