Happens with the normal sword guard template that has been upgraded with a material. When adding the trinket textures just disappear. Tested this with multiple different resources and trinkets.

Hmm, happens for me with everything. Tried it with only oak wood but gave the same result.

Right, I had it happen to me randomly. Hopefully its caused by ambiguity in the sword guard model definition.

Does this happen with vanilla materials, or only material added in a custom pack? I tried to replicate it, but I showed up fine for me with vanilla materials