Including 0 values when calculating STDEV

Anonymous Maybe something like this, PBIX is attached below signature. I'm not getting your stated standard deviation values but I added a debugging measure into the PBIX that should help you determine what is going on. With the debugging measure you can see that it is returning 0's for empty months.

Measure StdDev = 
    VAR __Date = MAX('Table'[Date])
    VAR __BeginDate = EOMONTH(__Date,-6)
    VAR __EndDate = EOMONTH(__Date, -1)
    VAR __Table = FILTER(ALL('Table'),[Date]>=__BeginDate && [Date]<=__EndDate)
    VAR __Count = COUNTROWS(__Table)
    VAR __Missing = 6 - __Count
    VAR __MissingTable = 
        SELECTCOLUMNS(
            ADDCOLUMNS(
                GENERATESERIES(1,__Missing,1),
                "rmv",0
            ),
            "rmv",[rmv],
            "Date",[Value]
        )
    VAR __FinalTable = UNION(__Table, __MissingTable)
    VAR __Result = STDEVX.P(__FinalTable,[rmv])
RETURN
    __Result