It looks like changes to outcomes when grading an assignment attempt don't get stored against that specific attempt, unlike the actual grade which is.
Either that's by design: you can change outcomes for a student's assignment but these aren't supposed to correspond to the attempt in which case the way this is presented in the grading interface — having it next to the grade — is misleading.
Or the implementation of outcomes is missing support for assignment attempts. It certainly sounds like useful functionality to have and I'll add some detail to the Tracker issue but it really needs someone who knows Moodle's grading system to have a look and confirm if this is the case.
It looks like you can see changes to the outcomes in the Grades history report in the course's Grades reporting area. That shows the number corresponding to the scale (1.00, 2.00, 3.00), not the scale itself (Not Met, Refer, Met) which I think is a bug too (it looks like MDL-49898).