An odd problem someone may have already run into.

I created an outfit consisting of one new custom item (body), and two existing ones (feet and hair) from the Skyrim base esm, and the character preview shows them all. But in-game, the NPC wears only the custom body item. Is there a limitation on outfits that requires all pieces to be part of the same esp/esm?

I'll be cloning the standard items later to change textures, so it may become moot, but I'd like to know if there's an unwritten rule here. That's also why it's a fresh topic, and not in Quick Questions, where it may get buried and hard to search.