Hi Raj
I read the entire thread again and I have noticed that I missed a very important information.
You have mentioned that the tab "Action Control" has been created for the firmed planned orders, therefore, I believe that you are using function module MD_SET_ACTION_PLAF to run mass changes on a planned order. See the following link for more details:
Mass Changes of Planned Orders (Action Control) - Procurement Proposal (PP-MRP-PP) - SAP Library
This kind of change will also lead the firmming indicator to be set, therefore, you should check who is making such changes.
You may use transaction CODE_SCANNER to check if there is any custom program calling MD_SET_ACTION_PLAF or check the user who last changed the planned order on table PLAF.
BR
Caetano