Cumulative sum

Add this measure or club: I still not understand the intention ... 

Sales  for Actual = 
CALCULATE(
	SUM('salesdata'[Sales ]),
	'salesdata'[Actual/forecast] IN { "Actual" }
)

Sales  for Forecast = 
CALCULATE(
	SUM('salesdata'[Sales ]),
	'salesdata'[Actual/forecast] IN { "Forecast" }
)

CT Actual or Forecast = 
SUMX (
       CALCULATETABLE(  values(salesdata[Week ]), salesdata[Week ] <= MAX(salesdata[Week ])) 
      , COALESCE( salesdata[Sales  for Actual], [Sales  for Forecast])
)

If you dont want multiple measures, you can club the measure logic into one measure!

Hoep this helps!

I do NOT see any issue.

Check these screenshots:

a) No selection

b) with selection of Week 2, week 2

b) with selection of Week 1, Week 4