I can’t reproduce this on my computer, so maybe you can help me out here.
First off, I just want to be clear that you’re editing the category topics here and not the categories themselves right? Your screenshots are of the topics. You edit the category using the “Edit Category” button.
Having said that, I have a sub category set up here, and when I edit its topic, the selected category that the topic belongs to is correct, not “(no category).” - That might actually be an installation issue as I can see the “iOS” category elsewhere on the page and that shouldn’t show up if it has a category.