Creating a new ship-to using 'Add Customer' from Order Entry Form (OEXOEORD)

I have a strange situation that is occurring with one of our customer service reps.  When she manually enters a new sales order, and uses the submenu that is available from the order entry form (right click on the mouse to see what I mean) to create a new ship-to, periodically, the ship to disappears.  This only happens with her and not all the time, but with the same customer.  It does not happen to the other 6 sales reps that enter orders.

I have checked the defaulting rules for customer, we are using those straight out of the box.  We do have a custom for warehouse, which is based on the customer ship to address, and this when she discovers the ship to disappears because this defaulting rule fails upon booking the order.  She then has to then create a new sales order, re-enter the information again, and it will work fine. I have tried a few combinations myself to recreate the problem to no avail.  I was trying to see if there was validation issue with postal codes(we use Vertex), where