Looks like graves is not compatible with Trinkets API or Cosmetic Armor (which is using trinkets API)

If player dies while having trinkets equipped, the trinkets do not get placed in the grave, and ultimately get lost even after retrieving items in grave.

I have multiple mods that use Trinkets API and all of them get placed in the grave upon death. In fact, Universal Graves is, to my knowledge, the only mod that does it on 1.17.1.

Yep, Trinkets is fully supported. Which version do you use?

Closing as no information was given