The mod seem to be really not liking mob heads being rendered ub GUI or first-person hands, if not tile entities entirely. In short, in my resourcepacks (which are not public anywhere, I'm sorry).
I have some replacements for the mobs and their heads, also including player. And it seems to be rather a rendering problem of some sort, because in some cases it renders them as expected, but in some it just makes them rather invisible or partly black. In case of mob heads, they just disappear whenever you highlight them or... highlight some other specific tile entity items? Also when grabbing specifically the trident in your hand, their inner parts are just going... black...
In case of player models which I have fully modelled as little "chibi players", they have some custom conditional animations, but with this mod they just change their poses at random moments because I made them animate differently in different conditions, mostly by stacking one inside each other like "if(is_in_gui, true1, if(is_in_hand, true2, if(is_on_head, true3, if(is_in_item_frame, true4, false))))". But for this mod these animations just kind of... randomly stack together, especially considering that for the most part they're slightly raised upwards, which should only be the case when you wear them on your head (they're just sitting on top of your head), slightly floating to left and right, which should only happen in GUI, and also frequently changing poses between general "sitting" one and the one that should be when held in the hand (the animation of them "climbing" on your arm). I think in this case it's just a problem of missing predicates or not exactly following them how it should.

Here's how it usually looks with Optifine. No rendering errors, no animation stacking, sorry for low quality, GitHub kinda made me do that.
https://github.com/Traben-0/Entity_Model_Features/assets/82406050/4f5027c3-2230-408d-9b07-7c2f365580a7

Here's how the same packs currently look with your mods.
https://github.com/Traben-0/Entity_Model_Features/assets/82406050/6fc349d6-4221-4f55-828b-458fd2a12f8f
https://github.com/Traben-0/Entity_Model_Features/assets/82406050/a4e2b2c7-f226-447f-8491-80597698629c

• Minecraft Version: 1.20.1
• EMF version: 1.3 (latest current version)
• ETF version: 5.2.3 (latest current version as well)