I found out that using the sol visage (and maybe the other masks) and doing a custom animation the mask will also show up below the player upside down

uh its not happening at all for me anymore but I did find out that 3d skin layer thing wont show during a custom animation

Even with your other mods uninstalled?

I suspect this is a compatibility issue. Does this happen with just Mowzie's Mobs and geckolib installed?

I have not tested that but I will try it

No it does not happen with just Mowzie's Mobs and geckolib installed

By the logic I know It still will happen because the custom animation doesn't account for the 3d skin layers