Why does my SO Line close automatically if a partial qty is added to a shipment ?

SOLVED

If my SO Qty is 30, but I have only 20 in stock.  The SO is set to a partial Delivery.

I create a shipment and pull the line.

The SO line is closed and I can't edit the shipment when 10 more are added into stock.

Is there a general parameter that dictates this behavior ?