Max Month -1

In your case MaxMonthValue calculation for "Max Month -1" subtracts 1 from the maximum year-month number found in the data that meets the condition [UsagesCount] >= 1. This works well in an unfiltered context because it works on the entire dataset.

However, when you apply a filter to a specific month, this changes the dataset that MaxMonthValue operates on, likely resulting in a different maximum year-month number or potentially no suitable data at all after the subtraction. This is especially true if the filtered month is the actual maximum month in your dataset since subtracting one would point to a month that's not included in your filter context

Max Month -1 Usages =
VAR MaxMonthValue =
    CALCULATE(
        MAX('Date'[Year Month Number]) - 1,
        ALL('Date'),  // Ignores the filter context for this calculation
        'Date'[UsagesCount] >= 1
    )
RETURN
    CALCULATE(
        [Total Kwh],
        'Date'[Year Month Number] = MaxMonthValue
    )