If you want to track the cumulative average by each new month this year, you can add a visual-level Year filter to this matrix and set value is 2021. I am not sure when you want the cumulative average, what should be the denominator to divide the cumulative total? Assume the denominator is cumulative number of months, you can create measures like below to achieve the results.
Cumulative Total = CALCULATE(SUM(Tbl[Count]),FILTER(ALL('Date'[Date]),'Date'[Date]<=MAX('Date'[Date])))
Cumulative Months = DATEDIFF(MINX(ALL('Tbl'[Date]),Tbl[Date]),MAX('Date'[Date]),MONTH)+1
Cumulative Average = IF(ISBLANK(SUM(Tbl[Count])),BLANK(),DIVIDE([Cumulative Total],[Cumulative Months]))
If you only want to show the Cumulative Average column in the matrix, click the down arrow next to Month field and check Show items with no data. Here is a PBIX file for your reference.
Community Support Team _ Jing Zhang
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