Not sure what causes it, but using Fresh Moves with this does some funky things with the 3d skin layers.

Using fabric 1.21.4. Fresh Moves and the mod are both also up to date, as well as ETM and ETF

Nothing to do with 3d skin layers, Fresh Moves isn't updated to work on 1.21.3+. For now, other animation packs like Better Animations can be used.