I don't think this is the issue being reported here, but at least through WeBWorK 2.14 there has appeared to be a bug (in my installation, at least) that results in gateway tests not correctly processing grade requests for users with instructor privileges. We haven't had any issues with students grading tests, however.
It seems that the student's login credential is being lost in the submission, and the problem states are therefore not being retained. I think this would produce the behavior being seen. I can't come up with a reason why this would happen, however. I assume that the course is using WeBWorK's authentication; the next question is probably whether that is the case.