Hello,
a case of bad (re)use of the assignment activity highlighted inconsistent behavior.

As we knows, if a restrict access condition for a group is added on an assignment activity so that only students belonging to this group can access to the activity, then only the same students will be counted as participants of the assignment.
But if a student has submitted one work in an assignment and then don't match the restrict condition anymore, his work will remain counted as a submission, listed, and downloadable in the teacher interface whereas the student will not be counted as a participant.

More precisely, with instructions for testing :

CASE 1 : group change
In a course with 2 students enrolled, e.g. "Alice" and "Bob".
1. As the teacher, create 2 groups, e.g. "Blue assignment" and "Red assignment".
2. Add Alice and Bob in "Blue assignment" group and nobody in "Red assignment" group.
3. Create an assignment with a condition that only allows "Blue assignment" group to access.
4. As the students, submit a work with Alice and with Bob.
5. As the teacher, enter the activity to check the counts : you should see 2 participants, 2 submitted, 2 needs grading (OK). 
6. In "View all submissions", check the list : you should see the 2 students and works (OK).
7. Click on "Download all submissions" : you should get the 2 works (OK).
8. Back in the activity settings, change the group in the "Restrict access" to only allow "Red assignment" to access.
9. Enter the activity to check the counts. 
    - Expected result : 0 participants, 0 submitted, 0 needs grading.
    - Reality : 0 participants, 2 submitted, 2 needs grading.
10. In "View all submissions", check the list. You should see no participant and no work (OK).
11. Click on "Download all submissions".
    - Expected result : you should not see the "Download all submissions" at all, like when there is no submitted work.
    - Reality : you get the 2 works.
    
CASE 2 : student removed of the group
Repeat step 1 to 7 of CASE 1.
8. Back in the activity settings, remove Alice of the "Blue assignment" group.
9. Enter the activity to check the counts. 
    - Expected result : 1 participants, 1 submitted, 1 needs grading.
    - Reality : 1 participants, 2 submitted, 2 needs grading.
10. In "View all submissions", check the list. You should only see Bob and his work (OK).
11. Click on "Download all submissions".
    - Expected result : you should get only Bob's work.
    - Reality : you get the 2 works.

This is not consistent : if participants are not counted, their work should not be counted, nor should it be offered for download.
However, the corresponding data should remain in base like for unenrollement/(re)enrollement (when a student is unenrolled, his work disappears in all counts and downloads of the assignment participations & submissions but (re)appears when the student is enrolled again).

What's your thoughts ?
Regards