UPDATE 2018-11-18 15:06 : I was going to include screenshots and now COULD NOT REPLICATE this problem - Calendar responded correctly to group override priority changes. PLEASE CLOSE THIS AS "NOT A BUG" - I cannot do this or delete this entry now. Thanks and apologies for the confusion.
Scenario (adapted from QA test
- Log in as the teacher and create an assignment with a due date set.
- Create 2 groups in the course and put a student in BOTH of them.
- Add two different due date overrides - one for each group.
- Use the arrow icons to make sure the earlier due date override has a higher priority than the later due date override.
- Login as the student: Timeline and assignment will correctly report the prioritized earlier due date. However, Calendar will INCORRECTLY show the lower priority later due date. Perhaps it automatically takes the latest due date override and ignores the teacher group override priority settings (which is Assignment's unique feature, so far).
So far tested on Moodle 3.6 master only.