FIltering max date

Of course, it returns everything. FILTER is an iterator (do you know what that means?). This code

    FILTER (
        ALL ( 'June WO`s'[Period] ),
        'June WO`s'[Period] = MAX ( 'June WO`s'[Period], 'June WO`s'[Period] )
    )

iterates all distinct values of 'June WO`s'[Period] and for each row calculates the condition:

'June WO`s'[Period] = MAX ( 'June WO`s'[Period], 'June WO`s'[Period] )

What's the result of 

MAX ( 'June WO`s'[Period], 'June WO`s'[Period] )?

Well, it's nothing else than

'June WO`s'[Period].

And what's the value of the condition

'June WO`s'[Period] = MAX ( 'June WO`s'[Period], 'June WO`s'[Period] )?

Well, it's TRUE. ALWAYS, since 

'June WO`s'[Period] = 'June WO`s'[Period]

Hence, your filter does not filter anything. It returns all distinct values of 'June WO`s'[Period]. To understand code, I'd suggest you learn something about DAX.

Your code should probably be this:

CurrentWO =
var __maxPeriodInCurrentContext = MAX( 'June WO`s'[Period] )
var __result =
CALCULATE (
    SUM ( 'June WO`s'[Journal Amount] ),
    'June WO`s'[Period] = __maxPeriodInCurrentContext
)
return
    __result

Please learn DAX. If you don't, you'll be producing numbers you will not be able to understand and will not know where they come from.

Best

Darek

Best

Darek