I followed the instructions to my eye model + eyelashes, and I can see it worked in the game, but somehow the model is a pixel "higher" than the original skin's eyes. I saw some players had a similar problem, but couldn't find the solution myself. Please help!

Found a fork from erinsc in the comment that fixed this but the eye problem still exists. Tested on 1.21.3 which has the outer layer problem.
Link to fork by erinsc

Videos showing movements

Original
https://github.com/user-attachments/assets/9f816a51-96db-404e-a968-901a88539328

Forked fix
https://github.com/user-attachments/assets/ced4d0df-aeac-4b75-90a3-1e51ffd58d9b

Seconding this tested both in 1.20.1 and 1.21.3, also including from other issues opened talking about the outer layer problem.
Tested with no other texture packs active but also tried fresh animations and both works fine together. though the eye pixel and outer layer problems still do exists.

TL;DR

• 3d skin layers and first person model mod works fine and doesn't conflict
• both 1.20.1 and 1.21.3 has the one pixel showing below the eye problem
• only 1.21.3 has the outer layer bug, 1.20.1 is safe from it
• both the eyeless and expressive version has the outer layer problems on 1.21.3

Modlists

(1.20.1)

• entity_model_features_fabric_1.20.1-2.2.6
• entity_texture_features_fabric_1.20.1-6.2.9
• essential_1-3-5-5_fabric_1-20-1
• fabric-api-0.92.2+1.20.1
• firstperson-fabric-2.4.6-mc1.20.1
• iris-1.7.5+mc1.20.1
• lithium-fabric-mc1.20.1-0.11.2
• modmenu-7.2.2
• skinlayers3d-fabric-1.7.4-mc1.20.1
• sodium-fabric-0.5.11+mc1.20.1

1.21.3 has the same mods but just updated and are the latest available on that version currently, also removed 3d skin layers for testing it out in 1.21.3

Screenshots

My edited skin

1.20.1 (has 3d skin layers mod)
without the resource pack and how it should look like

one pixel below eye problem

1.21.3 (with no 3d skin layers mod)
outer layer problem (eyeless version)

outer layer problem + one pixel below eye problem (expressive version)

Hey, I think I found a solution for the eye-problem! If you have eyes on the lower mid, you should use the "bottom eyes" model. Somehow this model also goes a pixel higher than needed and just on the level the lower-mid eyes require.

Still not sure what to do with your outer layer problem or the lashes, but still... At least eyes are okay now. I will update if I figure something out

UPD: Eyebrow placement works fine, so if you want to use eyebrows instead of eyelashes, you should place them as needed in lower-mid section, not bottom one