IT 31 should be maintained which shows all pers.no's for the employee.
Also IT 121 to show the priority of pers.no's.
see link
Central person infotype during hiring action | SCN
Infotype 0031 (Reference Personnel Number) | SCN
Employee - Personnel & Organization - SAP Library
Person ID for contract employees | SCN
Please explain what you mean by Personnel Assignment drop down? Are you referring to the person id's relationships in PP01 txn? All the relationships for the person id should be available, if PD-PA integration is active