Create an hidden category in Moodle 3.3 and a course inside it.

Using Boost or Clean theme the hidden category is visible in the breadcrumbs, even if you login as a student.

Using clean theme, the category is visible also inside the navigation block. 

When clicking on that hidden category link an error is generated.

FULL STEPS:

1) create an hidden category

2) create a visible course inside the hidden category

3) as a student go to the course (without being enrolled)

4) you will see, in the breadcrumbs, the hidden category

4) click on the category

5) you'll get an error