Hi,
Please check the note: 457110 - FAQ: Source determination in purchasing - Question Number 11 and the note: 206684 - MRP: automatic source determination , 550014 - FAQ: Source determination in material requirements planning - Question 1 & 2, which explains the system design for automatic source determination in MRP.
If you need a different behavior, you may use the exit: EXIT_SAPLMEQR_001
Regards,
AKPT