We have the same issue, and when you try to navigate away from the page, it asks us if we want to leave the page because we have unfinished changes.
We have also noticed that when saving a grade, the last saved date is blank back on the submission screen (showing all students). Does this happen to you as well?
Whilst there is a grade recorded, it doesnt seem to finish the process, so the various reports we run think it's not marked.
It's causing us a huge loss of productivity, confusion amongst staff and students. Please let me know if you work out what is happening.