Same DAX Calc - diiferent answer: Import v Direct Query

Hi, I have created a market share calculation:

Market Share =
DIVIDE (
    SUM ( 'Sales Invoice'[Invoice Amnt (LCY)] ),
    CALCULATE ( SUM ( 'Sales Invoice'[Invoice Amnt (LCY)] ), ALL ( Customer[Customer.Customer] ) )

)

This works perfectly in 'Import Mode' but when i recreate the dax in 'DirectQuery' the second part of the dax doesnt work - it returns the sales value for the filtered customer. therefore returns 100% market share.

I have check the tick box for 'allow unrestricted measures in directquery mode' but still no luck.

Is this a limitation of directquery?

Thanks,

Chris

ps - my table names are slightly different in my warehouse & cube.