The current use of "Group Mode" and "Grouping" settings in Assignment implement unexpected behaviour related to teachers only being able to mark students in their groups.
All other modules use these settings as specified in the help "If a grouping is selected, students assigned to groups within the grouping will be able to work together.". I would love to reclaim these settings for sanity and remove the "Grouping for student groups" setting.

If you agree/disagree, please vote and comment on https://tracker.moodle.org/browse/MDL-46041.