Function to add lines to a direct delivery sales order and update PO

We are running AX 2012 R3 RTM currently and so far we have been able to figure out that removing a line on a SO that is set to direct delivery the PO is easy to update as well. But not the other way around.

If we add lines to the orignal SO after the PO and SO are linked as direct delivery, the SO is updated, but not refelcted on the PO.

So either we´ll have to add them manually mand mark them manually as well to get the correct reservations and direct delivery or we´ll have to delete the original SO+PO and re-create them from scratch.

I´ve not been able to understand if we are doing something wrong, or if the functionality is not supported in our version or if it´s available in any newer version or not.