Error HRC-1035539 when manually updating Grade against Position

Summary:

When manually updating Grades against position we are receiving this error on several positions. "The values 30000000xxxxxxx are not valid for the attribute WorkTermsAssignmentId. (HRC-1035539)"

Content (required):

We are manually updating Grades against active positions via the Position Details screen. For the most part, they were successful but a few are not allowing update and giving the error above. The records are all Terminated Contingent Workers. What would be the best way to identify what is causing the error? It seems one of the errors is the same worker but on two different effective assignments. Any guidance would be greatly appreciated! Thank you!