Turn off the Aggregate only non-empty cells function in your gradebook. It is in the gradebook under Categories & Items > Full View. Uncheck the box in Aggregate only non-empty grades column.
Happy 
oodling!
That's beyond my expertise then. I'm just a GUI admin. Perhaps Tim or one of the other big Moodle coders will jump in with advice. Good luck
Happy oodling!
You may have done this already but check to see if all the permissions for these role(s) are set correctly. Check at the system level, course level and also the quiz level to see if any of the quiz permissions have been changed.
Are other users at the same role having the same issue?
Happy oodling!
At my university, we stress to grade at the local activity level. The main advantage I inform my faculty of doing so is that they then only see submissions and grades of that assignment. They won't get distracted by seeing all the grades from other assignments. In addition, the submission is stored at the activity level so there is no reason to read the submission and then take extra clicks just to get to the gradebook.
An analogy I use when teaching gradebook workshops is to describe the gradebook as the federal government and the activity as the local goverment. The federal government doesn't make all the laws; it leaves many decisions up to the local government. (E.g. person wouldn't go to federal level to complain about trash in local park; they would go to local government for help) The federal level always overrides the local level though.
When there is no grade entered yet, that is like neither government has passed a law on it yet. When you enter a grade in the local activity, this is pushed to the gradebook. Since there was nothing in the gradebook/federal level to begin with, the grade sticks in the gradebook. If after entering a grade in the activity/local activity, you then go directly in the gradebook and enter a grade, then the one in the gradebook will "override" the local activity grade. (E.g. Supreme court overruling city council on how to keep park clean) Since there is now a gradebook/federal level grade, what the activity/local level had entered is ignored and nothing new can be added at this level unless this override is removed. To turn off overrides is then a big pain because it is like Congress having to go back and review the laws and make changes.
Happy oodling!
I believe you have to either delete the old assignments or switch them to a point value/another scale. This way they are no longer associated with that scale. The latter will lose the grades you have associated with them though. So perhaps back up the course and submissions if you still need them, then delete assignments and then delete scales.
Happy oodling!