calculation error data sumx

ivannye , Check the datatype of EndOfMonth, That can cause this issue. Over all formula seems correct, Try this small change

Running12months 1 =
var currentdate=maxx(allselected(HeadCount),[EndOfMonth])
var yr= YEAR(currentdate)-1
var mt = MONTH(currentdate)+1
var previousdate=DATE(YEAR(currentdate)-1,MONTH(currentdate),DAY(currentdate))
return
SUMX(FILTER(ALLSELECTED(HeadCount[EndOfMonth],HeadCount[Costcentre]),HeadCount[EndOfMonth]>=previousdate && HeadCount[EndOfMonth]<=currentdate),[Empoyees That Left 2])