It appears that the group submission settings:

Students submit in groups
Grouping for student group

gets locked and not editable when a teacher (who is not a member of a group) clicks on the assignment of a student and clicks "grade" (even without saving).

Replication:

1. Create a course.
2. Enrol students 4 students and 1 teacher
3. Create two groups: grpA (student1 and student2) and grpB (student3 and student4).
4. Create a grouping: groupingABC (grpA and grpB)
5. Leave the teacher with no group.
6. Turn edit mode to On.
7. Add an assignment activity.

• group submission settings
•  Students submit in groups  = yes
• Require group to make submission = yes
• Require all group members submit  = no
• Grouping for student groups = None (note that this is a select picker control)
  8. Save and display.
  9. Do not click "View all submissions" and do not click "Grade".
  10. Log in as the teacher user.
  11. Go to the assignment. You will notice the message:
  "The setting 'Require group to make submission' is enabled and some users are either not a member of any group, or are a member of more than one group, so are unable to make submissions."
  12. Do not click "View all submissions" and do not click "Grade". Just click Settings and confirm verify 
  Students submit in groups = editable (can change value)
  Grouping for student groups = editable (can change value)
  13. Go back to the assignment and click "View all submissions".
  14. Click "Grade" to one of the students. Do not do anything, do not save. Just go back to the assignment page.
  15. Note that up to this point, there are no submissions.
  16. Click settings for the assignment

RESULT:
Students submit in groups = yes (but locked, and cant be changed) 
Grouping for student groups = none (but locked, and cant be changed)