I've got both Accessories and the Trinkets compat layer installed, however regardless of whether i have an accessory in a main slot or not, nothing placed in a cosmetic slot appears on my player.
Fabric 1.21.1.

Things placed in regular main accessory slots appear fine. Issue is only seemingly with cosmetic slots.

I am having a hard time reproducing this issue, could you send a copy of your log and more info on the mod that is not rendering.

Since I have not gotten any response for more info I will be closing this issue until more info is given.