Additional assignment-related features and issues within Microsoft Teams for Education
By default, students added to the class after an assignment is created won't be able to submit their work or access attached resources...but for some reason they still see the assignment. Affected students also have those assignments grayed out in the Grades tab.
The workaround I've found is to edit the assignment and change the Assign To setting, selecting the bottom option ("Assign to all students added to this class since [assignment creation date].").