I did some digging on this, I believe this is system design since V6 Patch 6. You can't change the WOS if it's coming from MPS/MRP. However, you are allowed to change the WOS that you created manually.

CONTEXT:
From the Workbench (Global or Detailed)

DESCRIPTION:
Authorize the update only on WOS in process not coming from the MPS/MRP because it disturbs the linked WIP that are not updated and it puts into question the result of the MPS/MRP.

CHANGES:
Corrected.

Makes sense. Thanks for checking.