Restrict site by membership level (without redirection), doesn't work as expected in singular course and membership

### Reproduction Steps

+ Create a Membership
+ Create a Course
+ on the Restrictions tab select "Stay on page" for **Restricted Access Redirect**
+ leave the other settings as they are by default (Display a Message and Restricted Content Notice)
+ navigate through your dashboard -> LifterLMS (Settings) -> Membership
+ as **Restrict site by membership level** search and set the just created membership
+ save
+ create a lesson not part of any course (testing purpose only)
+ as visitor, in front, navigate to the Course created before
+ as visitor, in front, navigate to the Lesson created before

### Expected Behavior

+ I would expect to see no Course content and no Lesson content but to see the **Restricted Content Notice** message set (default) in the Membership created before

### Actual Behavior

+ the Lesson shows no content but the Course still shows its entire content.
Both display the **Restricted Content Notice** message set (default) in the Membership created before

### Error Messages / Logs

n/a

### System Information

LifterLMS 3.37.9
WordPress 3.5.3

### Browser, Device, and Operating System Information

n/a