I hope I'll explain this one clearly! :p
Guard villagers are not made by the same person on Neo vs Fabric. On forge they recently added the biome specific layer via a subfloder biome with a base texture in the guardvillagers texture folder (same logic than villagers). On fabric, there is just 6 textures in the texture folder.
On my guards I have their weapons on custom locations on the body rather than held in hand. To do this, I have the arms were the weapon is held with no cubes, and a set of fake arms.

For my maths, the fake arms follow the vanilla ones all the time, and the normal arms will follow the vanilla behavior if agressive, or have a custom location (set by variables).
For the armor to match the fake arms, I set some variables that just copy the fake arms that I call in the armor_layers.jem.
Here is a picture of the maths for better understanding.

This is working as I expected on Fabric, but on forge, the biome layer is displayed at the place of the arm with no cubes. The armor is also following this. What I don't fully get is the fake arms are still rendered with the base texture...?

PS, on here the sword is not held by the left hand, they are both right handed, and the sword is meant to be in a scabbard.
(Also, for the math to work, the fake_arms have to reference left_arm, before left_arm actually uses the variables. But I think this is very normal?)

I have made sure my model was following the roots of the export (I have exported again from both mods), both are the same, and mine is the same minus the cubes deleted

I have other models where I don't reference left_arm, I use custom math everywhere. I does display correctly on both. But I've had still noticed some Z fighting sometimes on forge, with also a little giggle that gives 2 set of arms from time to time. (this is really rare and not easy to spot tho). This has to be for the same reason

Both guard villagers mods aren't updated past 1.21.1, so I tested on them with EMF 2.4.1/ETF6.2.9
(does the same on 1.20.1)

Here is a pack for testing:
for issue traben guard layer broken.zip

Model is guard22
Will be used by any guard with a sword in main hand if you stand away from 3blocks or more

Possibly related to #363 I opened few days ago.
I don't know any other mod that use this layer mechanic, would be interresting to see if the same issue occur

Working with villagers recruit wich also use layers for textures and the same thing happens.
But also always when the fake arms part references right_arm. The main texture is ok, but the layer will follow the custom location of the left

As seen here, the right arm has the green texture of "idle location", but the biome with red accent is displayed holding the sword

I will soon provide a example pack for better visualisation ;p
This should affect vanilla zombie villagers too.

Closing as I opened #387 instead, Better explained there and this is not related to guardvillagers mod only