I am getting the same types of problems.
I have just started trying to use the Assignment Activity, and am very disappointed.
When I try and filter for ´Not Submitted´I get users who have submitted their Assignments.
I also keep having to click the ´Reset Filter Button´(right hand side) to try and make the filters work.
Can anyone help?
I have been using a version of Moodle 2.7, which has just been upgraded to 3.1.