Yeah, I think this is one of those 
=1000 words situations.
I’ve got a zillion categories and subcats (it’s about 750 categories total, with about 30 at the top level–just how stupid that may be is a for another topic).
Here’s one of the categories with the subcats:
When I try to reorder categories, I get this:
See how all of those counties, which are sorted under the proper states all end up in one big list?
When I did the subcat sort I did them by 5s. My guess is that all of the ones in that image are the first county by alpha in the state.
What would be really nice is to be able to sort within a subcategory, but since there’s no category hamburger on a /c/category-slug page, that’s not an option.