I think your issue is with this:

MEMBER [Measures].[FID] AS

Head(

NonEmpty(

EXISTING [Time].[Fiscal Hierarchy].[FiscalMonth].Members,

([Measures].[JBA Sales Quantity])

),

1

).Item(0).properties("key", TYPED)

Since you are using .item(0) and HEAD,1 you are limiting the result to only one member. So when the user picks more than one brand it will always be the earliest FID that it can grab from the brand members. You need to integrate brand into your initial calculation rather than splitting it up as you are now.

Best of luck 🙂