Changing Shipping Method at Sales Order Line Revert Back to Default Shipping Method

Hi All,

We have an Issue for Shipping Method at Sales Order Line, 
Default Rule Setup : Header - Ship method taken from Ship to address ( e.g DHL)
                         Line- Line Level Shipping method taken from Sales Order Header Level ( Same as Header Ship method- DHL ) See attachment

Issue :  Once sales order Entered Header Level Ship Method cascade correctly at line.
             If user Change Line Level Shipping method From DHL to FedEx at Model Item Line it Correctly Cascade at subsequent line ( All line now display FedEx) If User Change Request Date at Model Item line Level with Check of Override ATP Flag and Save it. Shipping Method Automatically Change from FedEx to DHL, later if I change Same Method(FedEx) at Sales Order header level it ask me for Cascade at line After Click on ok Change Shipping Method not Cascade at Line.