Cumulative distinct count

I think the issue with your measure might be related to the way the ALL function is being used, particularly in the context of your slicers.

Try the following : 

Measure =
VAR CurrentFY = MAX('Calendar'[Fiscal Year])
VAR MaxMonth = SELECTEDVALUE('Calendar'[Fiscal Month])

VAR Results =
CALCULATE(
    DISTINCTCOUNT(Customer[CustomerID]),
    ALLEXCEPT('Calendar', 'Calendar'[Fiscal Year]),
    'Calendar'[Fiscal Year] = CurrentFY,
    'Calendar'[Fiscal Month] <= MaxMonth,
    FILTER(
        Customer,
        [Total Offers] >= 1
    )
)

RETURN Results