Cumulative not working correctly

You don't need to force the context transition, you have to break it!
First create a calculated column:

BreakContextTransitionTable = 
VAR __YMTable = SUMMARIZE(ALL('Date'), 'Date'[MonthnYear])
VAR __YMTableEW = ADDCOLUMNS(__YMTable, "@EW", [EW])
RETURN
__YMTableEW

Then add this measure:

EW = 
VAR __filter = FILTER(BreakContextTransitionTable, BreakContextTransitionTable[MonthnYear] <= MAX('Date'[MonthnYear]))
RETURN
SUMX(__filter, BreakContextTransitionTable[@EW])

Giving this result:

Of course you can break the context transition by rewriting the EW measure (removing all Calculate())