Measure returning same value for all rows in matrix

Your other column names are not shown in your pic, but try the expression below for your variable.  I assume you are using the Date_EndMonth column in your slicer, so you need to remove that filter (but not more than that).  If so, that is why ALLSELECTED didn't work; it would not remove the filter coming from that slicer.  Try just removing only the filter from that column (and if you have a sort column for it, remove that one too).

VAR Backlog_PreviousMonth =
    CALCULATE(
            SUM ( 'POC Data'[IFRS Backlog] ), ALL('POC Data'[MonthEndDate], 'POC Data'[MonthSortColumn]),
        'POC Data'[MonthEndDate] = Date_PreviousMonth)

Regards,

Pat