Sorry, I couldn’t delete my post in modelign section but I realised this was in wrong section before, so I’m reposting here.
"Ok this one is REALLY wierd…
So I had two objects that I joined together. I had the same surface applied to both. I then went to bake full render, and one of them bakes to uv map and other comes out black. Both are part of the same object as I said. This makes no sense to me at all.