I found a solution. When my new (duplicated) item wouldn't show up in the right category, I finally deactivated and then reactivated it. That fixed the problem, it's now in the correct category.
Never mind. Just created another new listing from a duplicate, and it was in the wrong category again, and I went through the process again - deactivated and then reactivated - but it's still in the category that I duplicated it from.
Okay, never mind again. It's now showing up in the correct category.