I made the category LangEx and then went to make a sub-category. So from experience we know that you first have to make the sub-category (really just a new category) then you can edit it to attach it to a parent.
So I made the new category then right away went to make a thread in it, thinking perhaps that if I took another action first then maybe the category would be ready to use as a parent.
When I made the thread, from the category page, it didn’t assign a category to a thread.
I’ve had similar issues with MediaWiki in that you make changes, but the jobs don’t get completed right away. I think this is happening when Categories are being created - there are jobs not being completed right away. Make sense?