Quiz not marked as complete when a question is manually graded

Quiz with 37 automatic questions and 1 which must be marked by the teacher.
Activity and course completion are turned on. Activity completion is based on the passing grade of 90%. Student tries and fails. Teacher goes in and marks the final question - they get 5 out of 5 for it and this means they've gone from a fail to a pass. Activity shows new grade. Activity completion does NOT flip them to complete.
Student passes - last question is graded and activity also does not complete.
Remove the last question which needs grading and activity completion works correctly.
Have had staff at a moodle partner test this and create a new quiz with 3 dummy questions then adding in one which has to be marked and they also have this problem