When a Rehire profile is created in EC, the Original start date in the employment details portlet will not be the same as the old employment’s Original Start Date.
SAP SuccessFactors Onboarding
This is expected system behavior currently. For Rehire with New Employment, Onboarding does not propagate any employment data in rehire cases.
In case you are attempting Rehire with New Employment from EC Directly - https://blogs.sap.com/2021/04/08/propagating-original-start-date-to-rehire-with-new-employment./
Rehire, Original Start date, not matching, propagate start date, New Employment, Employee Profile, Employment details, Onboarding , KBA , LOD-SF-OBX-RHM , Rehire Mechanism , Known Error