Cumulative sum comes up as empty when there is no value for that month

I think the problem might be that if a row doesn't exist then the max start of year may not return a value.

Try using DATESYTD instead

YTD Cumulative Cost =
VAR YYTD =
    CALCULATE (
        [Total Cost USD],
        DATESYTD ( 'Calendar Table'[AWB Month Year], "31/10" ),
        ALLEXCEPT (
            'Air Freight Spend',
            'Air Freight Spend'[Dom/Int],
            'Air Freight Spend'[Origin_APC_Billing_region],
            'Air Freight Spend'[Origin_Country],
            'Air Freight Spend'[Origin_APC],
            'Air Freight Spend'[Lane],
            'Air Freight Spend'[Origin - Destination Country],
            'Air Freight Spend'[Destination_Country],
            'Air Freight Spend'[Destination_APC],
            'Weight Band Order'[Weight Band],
            'APC Map'[CtrlRegion],
            'Air Freight Spend'[Suspicious Data],
            'Air Freight Spend'[Carrier_Mode(s)],
            'Air Freight Spend'[Carrier_Service(s)],
            'Air Freight Spend'[Cost_USD_Source],
            'Air Freight Spend'[Consolidated AWB Flag],
            'Air Freight Spend'[BillingRegion(s)],
            'Air Freight Spend'[BillingCountry(s)],
            'Air Freight Spend'[BillingCenter(s)],
            'Air Freight Spend'[Airli],
            'Air Freight Spend'[Shipment_ID(s)],
            'Air Freight Spend'[CustomerName(s)],
            'Air Freight Spend'[Parent(s)],
            'Air Freight Spend'[Grandparent(s)],
            'Air Freight Spend'[Type(s)],
            'Air Freight Spend'[Account_Service(s)],
            'Air Freight Spend'[Involves_SAM_Active_Flag],
            'Air Freight Spend'[Involves_KAM_Active_Flag],
            'Air Freight Spend'[Involves_Dangerous_Goods?],
            'Air Freight Spend'[Involves_Payload?],
            'Air Freight Spend'[Temperature_Control(s)],
            'Air Freight Spend'[Temperature_Grouping(s)],
            'Air Freight Spend'[PJT(s)]
        )
    )
RETURN
    IF (
        MAX ( 'Calendar Table'[AWB Month Year] ) >= DATE ( 2022, 10, 1 ),
        YYTD,
        BLANK ()
    )

Rather than specifying the code again, I would have a base measure as e.g. [FYTD] then your variance would be

FYTD vs Previous =
VAR CurrentYear = [FYTD]
VAR PrevYear =
    CALCULATE ( [FYTD], DATEADD ( 'Date'[Date], -1, YEAR ) )
VAR Result =
    DIVIDE ( CurrentYear - PrevYear, PrevYear )
RETURN
    Result