There was a change to Assignment module somewhere back around 2.2 -> 2.3 and beyond? (don't re-call now it's been so long ago). There is an Assign Assignment module (the newer) and the old Assignment (2.2). Check your Site Admin menu for a 'Assignment Upgrade Helper' at the bottom of the site admin menu.
Click it ... convert any assignments it list as old to the new. If there are bunches of them, might want to select fewer number to convert at one time - lots of processing going on there.
IF all old assignments convert upwards ok, then disable the Assignment 2.2 you see in
site Admin -> Plugins -> Activity Modules -> Manage Activities.
Teachers/Couse Designers will no longer have an Assignment 2.2 option in the creation of an Assignment (ie, the older version). That might then fix the issue.
As is always a good practice, suggest making a code and DB backup of the site before doing anything/something that's system wide (just in case)
'spirit of sharing', Ken