What we did is:
Change the validate of "Prod. Order Line No."
...
Case "Entry Type" of
"Entry Type"::Output:
Begin
"Routing No." := ProdOrderLine."Routing No.";
"Routing Reference No." := ProdOrderLine."Routing Reference No.";
"Unit of Measure Code" := ProdOrderLine."Unit of Measure Code";
"Location Code" := ProdOrderLine."Location Code";
"Bin Code" := ProdOrderLine."Bin Code";
validate("Variant Code", ProdOrderLine."Variant Code");
END;
End;