Hello
Probably the flag "start in the past" was not checked on the customizing transaction OPPQ. In this case, system cannot create a replenishment proposal with the start in the past, therefore, it set the start date as today and calculates the end date based on the planned delivery time or in-house production time.
The picture below illustrates the scenario:
See the link Scheduling - Material Requirements Planning (PP-MRP) - SAP Library for details.
BR
Caetano