If you go to the grader report and turn on editing you can tab through the fields. With quick feedback it goes through a single assignment. student 1 grade, student 1 feedback, student 2 grade etc. Shift tabbing doesnt reverse this order. It goes student 2 feedback, student2 grade, student 1 grade (skipping student 1 feedback). Further shift tabbing causes a JS error to appear in the error log saying that parts is null.
var parts = regexp.exec(arg.getAttribute('id'));
arg exists but has no id attribute. I'd guess that the incorrect shift tab behaviour is causing arg to wind up pointing at the wrong node.