Restricting access to a course module by using "activity completion" and "Previous activity with completion" does not work, when activity completion of current module is disabled.

Steps to reproduce:

1. Create course.
2. As teacher create two activites, for example assignments (also tested with labels, seems to not depend on type of activity): assignment1, assignment2
3. assignment2 has to be directly below assignment1
4. assignment1: "Activity completion" -> "Students can manually mark this activity as completed"
5. assignment2: "Restrict access" -> "Activity completion" -> "Previous activity with completion"
6. Now the important part: assignment2 -> "Activity completion" -> "Do not indicate Activity completion"
7. Enrol a student in this course
8. Log in as student of this course. Mark assignment1 as completed. You will not have access to assignment2 -> Bug!

This does not happen, if you explicitely specify assignment1 as dependency for assignment2, it only happens if you use "Previous activity with completion".

Another strange thing:

If you mark activity1 as "completed" not as student, but by switching role from teacher to student and then change back to your teacher role, you will see activity2 labeled with "Not available unless: The activity (Missing activity) is marked complete (hidden otherwise)" -> this also should not happen.

Tested this on 4.0.1 as well as 3.11.6, versions 3.11.7 and previous versions probably also affected.