Okay, I tested and can reproduce the problem on my test website. The Posts work as expected, but indeed, the sub-categories fail.
One solution, which is really not practical if you have a large number of categories, but resolves the issue, is to create pages with the same paths and use a Shortcode in those pages. The Shortcode will display the corresponding pages as a list. The one problem is it won't use the same formatting as expected by default in your theme...
So if you have a parent category named
parent and a child category named
- create a page with the path set to
- create a child page with the path set to
/child/ and then mark it as a child of
Note that you can't get the correct path for the child page (
/parent/child/ in my example) without first creating the parent page. One good thing is that the look and feel of the list in the parent will thus have the same look and feel in the child.
In the pages, you can enter any text you'd like and then use a Shortcode such as this:
I use the Display Posts Shortcode plugin, which is probably not the best plugin available for that job, but it has the advantage of work well. Feel free to try with others...
Now if you have hundred of categories and add/remove many of them all the time, that's going to be a nightmare. If you have a few well selected categories, you should be just fine that way. (You'll also need a little CSS to help the output formatting.)
There is a screenshot of the child page. We see that the path looks correct. This is because I setup the parent (right side of screen). I also show the Shortcode.
Now, what you should probably do is report that problem as a bug at WordPress.org.