So that mod basically uses the Trinkets API to add a new armor slot specific for the elytra. But when you put a custom elytra in that slot it looks like a vanilla elytra, even though it should look like the customized one.

I have to mention that the mod works as intended when you put the elytra in the chestplate slot, there's no reason for it not to work lmao

Nevermind, switching to ElytraSlot mod worked fine. ElytraTrinket does not support modded elytras.