Key Figures in Joined Data Source

Hello,

I have created some Key Figures for the Data Source "Sales Order Item". Reports based on this Data Source are working fine and also my Key Figures are displaying the right results.

Now I want to create a Joined Data Source (Left Outer Join) with the Sales Order Item and Outbound Delivery Detail Line Items. The problem I have here is, that my own Key Figures based on Sales Order Item are not showing up in the list and I cannot select them.

Does anyone know why? Do I have to create the same Key Figures again and again as soon as I want to join the Sales Order Item Data Source with any other Data Source?

Thanks for any feedback.