Measure breaks join

I have two very simple tables that I am using to check this issue: a table of orders with value and sign (credit or debit) and a table of invoices with the ref order number and the sale value

I join them on order number

I created a Table visual with Order number from the Order Table and IORDNO from the Invoice table and sum the value in the Order Table.

Then I filter to exclude entries where the order number from the order table is blank (i.e. invoices that do not match any of the orders).

Which behaves as expected.

Then I create a measure: Measure = SUMX(orders,[Value] * [Sign])

and add this as a column in the table and the relationships is broken.

How can I have non matching order numbers in the Table when there is a join on this field and why does the measure cause this?