I created a category and I protected it so that only a (currently empty) group of people can “create / view / reply” inside it. As an admin, I can access the “category definition” topic.
If I copy that topic’s URL and I paste it in a new incognito window (so I visit the URL as a not logged-in user) I correctly receive an invitation to log in.
Now, I created a subcategory and I tried to do the same thing (copying and pasting the URL in a new incognito window). It doesn’t matter whether the subcategory is protected like its parent or not, the page that shows up is a blank page.
Note that if I check the source (CTRL+U in chromium) of that blank page I seem to be able to peek at the content of the topics.