Blanked Out DFF is throwing error when tried to create a requisition using existing Requisition

Summary:

We have modified the list of values on the approver field to show only the active user records, and deployed the flexfield. After that when we trie dto test by creating a requisition using the existing requisition. The Approver field having a inactive user got blanked out. but when we continued and tried to save and close or submit the requisition. The System thrown an error called Second approver XYZ is not in the List of Values. How we can troubleshoot this error when we have such cases when the old values blanked out the field and users will not