It appears that the original due date of an assignment is still displayed in a users calendar even if another one is displayed if there is a group override due date.

Replication steps

1. Create a course. Enrol 1 student.
2. Create a group and add the student as a member
3. Create an assignment and set:
name: MyWork
due date: 14 July
group mode = separate groups
4. Log in as student and view calendar.
5. RESULT: Correct entry. 14 July has an entry for the assignment
6. As admin add a group override for the assignment set:
Due date: 14 August
7. Still as student refresh and view July Calendar
8. RESULT: There is still an entry for 14 July for the assignment (original assignment due date)
9. Still as student refresh and view August Calendar
10. RESULT: There is an entry for the 14 August (override date for the group)
 
Note: Perhaps the original date of 14 July should not be displayed for the user as there is already an override of 14 August