Hi this sounds strange, but when a user wants to publish a listing in a main category (not a sub category) it doesnt give that option in the item post.
In admin i have this

Category settings

Parent categories
  Allow users to select a parent category as a category when inserting or editing a listing

And its ticked.

On the item publish page i have select category, then sub category, then sub-sub category, there is no option to choose main category

@aide
You must select default category selection style to work.

That was easy to fix !
Thanks