DAX: Running Total that doesn't Reset, with Inactive Relationship (pbix included)

Yes, this solved the problem because under the hood your DAX is transformed into:

VAR v1 = MAX('DIM Calendar'[Date])
RETURN
CALCULATE(
    [Count New Materials],
    // This filter overwrites what's
    // coming from the outside world.
    FILTER(
        ALL( 'DIM Calendar'[Date] ),
        'DIM Calendar'[Date] <= v1
    ),
    // If your table 'DIM Calendar' is
    // marked in the model as a date 
    // table, this ALL (in your code)
    // is not necessary since the engine
    // performs this line below automatically.
    ALL('DIM Calendar')
)

The other one does not work because it's equivalent to this DAX:

var MaxDate = MAX('DIM Calendar'[Date])
return
CALCULATE(
    [Count New Materials],
    KEEPFILTERS(
        'DIM Calendar'[Date] <= MaxDate
    ),
    ALL('DIM Calendar')
)

KEEPFILTERS prevents the expression from reaching rows outside the current filter context and you need to be able to do it to calculate what you want.