I just tested this on my forum and here are the results:
I created a category that had access to staff and TL3, and then a sub-category for staff only.
I created one topic using the TL3 category with a test account, then three with my main admin account. Results below.
This is what is on the TL3 side who can see the parent category, but not the private subcategory:
Here is what it looks like on the staff/admin side:
And after asking @JammyDodger I have moved this to Bug 
Have a great weekend!