a STO process is within 2 parties in your own system
if pant A ships 100 and plant B receives 50, then still 50 are in-transit.
You can't just leave it there. A delivery complete indicator cannot clear in-transit stock.
with external vendors, you do not have any in-transit stock. if you receive less than ordered then the delivery complete indicator closes the order virtually, hence you get no item proposal in MIGO. However this message is not really an error, you can still do a receipt, you only need to set the flag for propose all items in the settings in MIGO transaction.