We had a lot of trouble firmly pinning this down because we were also seeing the issue identified in MDL-78664.

Group override dates display correctly in all places (the quiz/assignment itself, calendar, calendar block, timeline) UNLESS a user is enrolled elsewhere on a site in a non-student role.

If a user is enrolled as a student in a course AND a teacher role elsewhere, they will only be able to see their correct group override date WITHIN the activity itself. The calendar block, upcoming events block, and timeline will show the activity's base due date.

Steps to replicate at https://sandbox.moodledemo.net (tested both the latest version 4.3 and version 4.1.):

• Login as teacher and access My First Course
• create a group 'Group 1' and add Sam Student into the group 
• create an assignment and give it a due date (same steps apply to quiz close date)
• add a group override to the assignment that extends the date. screenshot 1.
• logout as teacher; then login as student, access My first course
• view the assignment page and confirm that the due date shows the date set in the group override and that the same date (along with the group info) is shown in upcoming events and calendar. screenshot 2.
• logout as student, login as teacher
• Access My Second course, enroll Sam Student as 'Non-editing teacher'
• logout as teacher, log back in as student
• Access My first course - observe that within the assignment, the due date still matches the correct group override date; at the course-level, the calendar block and upcoming events block show the base due date. screen shot 3.