Did you remove everyone else from the Assign To box such that only the individual student shows? I have had this happen before. To fix, add everyone back into the Assign To box and their grades will come back into the gradebook. This has worked for me in the past.
To add a new/different due date for individual student, click the + Add at bottom of Assign To box. Then select that student's name and enter new due date. Now there will be 2 Assign To boxes: one for the student and another for Everyone Else.
https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-assign-an-assignment-to-an-individual-s...
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