Layout based on Invoices Table in List view will show only and all records in Invoices Table (by default - can be changed)
Fields from Invoice Data Table can be placed on this layout, but show only first related Invoice Data record.
Portal showing Invoice Data can be placed on form layout in Invoices, creation of new record in Invoice Data can be enabled or not.
A Join Table connecting Invoices and Invoice Data is often created to make reporting easier.
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For some reason I don't understand - it's working now.
The layout is based on Invoice Data table. Invoice Data has a relationship to Invoices.
The Add Item is working too.
I'm puzzled but happy.