Calculating cumulative backlog with multiple instances per order

You can create a measure like

Backlog =
VAR currentDate =
    MAX ( 'Date'[Date] )
RETURN
    SUMX (
        'Planning',
        IF (
            SELECTEDVALUE ( 'Planning'[Deadline] ) > currentDate,
            0,
            VAR currentQty =
                SELECTEDVALUE ( 'Planning'[Qty] )
            VAR totalProduced =
                CALCULATE (
                    SUMX ( RELATEDTABLE ( 'Production' ), 'Production'[Qty] ),
                    REMOVEFILTERS ( 'Date' ),
                    'Production'[Date] <= currentDate
                )
            RETURN
                currentQty - totalProduced
        )
    )

Not sure why its not working at the aggregate level, but I was able to get a working aggregate with

Total Backlog = IF( ISINSCOPE( 'Planning'[Order ID]), [Backlog],
var summaryTable = ADDCOLUMNS( SUMMARIZE( 'Planning',Planning[Order ID]), "@val", [Backlog])
return SUMX( summaryTable, [@val]))