I have found a way to finally replicate why some group overrides for assignment due date does not appear for Calendar events/ Upcoming events/ Timeline.

See below:

Environment:
$version  = 2023042401.04;
$release  = '4.2.1+ (Build: 20230629)';
 
Replication:
Variation1
1. Create a course (courseOne) and set Groups = Separate Groups
2. Enrol a user (teacher1) and assign role = Teacher
3. Create another course (courseTwo) and set Groups = Separate groups.
4. Enrol teacher1 and assign role = Student
5. Still at courseTwo, add an assignment activity.
Name: assignmentC2
Due: 13 July 2023
6. On another browser/session, log in as teacher1.
7. Visit your dashboard page.
8. RESULT: Calendar shows "assignmentC2 is Due" entry on 13 July. This is correct.
9. As admin go to courseTwo, create a group (grpC2) and add teacher1 to it.
10. Go to assignmentC2 and add override group grpC2 and set Due 13 August 2023.
11. As teacher 1, refresh your Dashboard and go to month of August for the Calendar.
12. RESULT: There is NO entry for the override date of 13 August 2023! – This is the issue/bug as there should be an entry for 13 August 2023.
13. Still as teacher 1, go directly to assignmentC2. The following is displayed. Due: Sunday, 13 August 2023. 12:00 AM. This is correct as the override date is 13 August.
14. Create grpC1 on courseOne and add teacher1.
15. As teacher 1, refresh your Dashboard and go to month of August for the Calendar. Note, NO entry again for 13 August
14. Delete the enrolment of teacher1 on courseOne.
15. As teacher 1, refresh your Dashboard and go to month of August for the Calendar. Note, NO entry again for 13 August
16. Add back the enrolment of teacher 1 on courseOne.
17. As teacher 1, refresh your Dashboard and go to month of August for the Calendar. Note, NO entry again for 13 August
 
Variation2 (sequence/continuation from above)
1. create a new user teacher2.
2. Still using courseOne and courseTwo.
3. As admin go to courseTwo and enrol teacher2 as Student and add to grpC2.
4. Log in as teacher2 and visit Dashboard/ Calendar.
5. RESULT: As teacher 2, refresh your Dashboard and go to month of August for the Calendar. Note, THERE is entry for 13 August
6. Go directly courseTwo assignmentC2. The info is displayed: Due: Sunday, 13 August 2023, 12:00 AM.
7. As admin go to CourseOne and enrol teacher2 as teacher role there.
8. As teacher2, refresh your Dashboard and go to month of August for the Calendar. Note, NO entry for 13 August. Remember that in step 5, there was an entry for 13 August
9. As admin add teacher2 to grpC1 on CourseOne. Note, teacher2 has a teacher role and now a member of a group.
10. As teacher2, refresh your Dashboard and go to month of August for the Calendar. The entry for 13 August is  VISIBLE. Hovering your mouse shows "assignmentC2 -grp C2 (Due date) for 13 August.
 
NOTE: If you remove teacher2 from a group in courseOne (grpC1), refresh the dashboard, the entry for 13 August will be removed. Adding back to a group will re-display the entry.
 
Take note that in variation1, there is no work-around the entry does not show even if you add the user to a group or even delete the enrolment. The override date is still not displayed.
 
Compare it to variation2, where there seems to be a work-around to add the user to a group in the course wherein the user is a "Teacher". It fixes the issue and calendar entry for the override is displayed.