Hello
On a reorder point MRP type, system always schedules the planned order using forward scheduling considering today as the start date, even if you are considering external requirements. See the following links for more details about the forward scheduling on reorder point:
Forward Scheduling for External Procurement - Material Requirements Planning (PP-MRP) - SAP Library
Forward Scheduling for External Procurement - Material Requirements Planning (PP-MRP) - SAP Library
If you want to have the planned order finish date equal to the requirement date, then you should use MRP type PD instead of a reorder point procedure.
If you need to use reorder point, you may use BAdI MD_PLDORD_CHANGE to adjust the dates. See this document for details about the BAdIs available on MRP.
BR
Caetano