The NPC isn't configured to use skin overlays

[squili]

When sending out the entity metadata for the NPC, the skin overlay isn't set. This means that things like hats don't render on the NPCs. This can be fixed by adding a byte with value 0x7F and index 17 to the metadata packet.
See: packet metadata

[keksgauner]

I didn't know anything about this.
Did you mean something like this?

LivingEntity le = (LivingEntity) npc;
le.getMetadata("0x4D").get(17);

I think I need to read about this

[squili]

I'm not exactly sure what NMS code you'd use for this. I wasn't able to get any NMS changes in the library to work on my test server, so I wasn't able to make a PR for this. All I know is that, according to the wiki.vg docs, that's what you need to add to get it to work.