Previously both Outline and Complete reports were showing information for any activity, whether hidden or not, to the users who had permission see hidden items.
So, usually teacher or admin would access those from student profile and review participation in any activity.
I can confirm that this was working on 2.7.17
Now the reports do not show stats for the hidden activities to anyone, including teachers and admins. Tested on 3.1.3
it seems to be a loss of functionality?
Please let me know if I should provide further info.