impossible to change the IF status from packed to shipped

Hi everyone, 

we face an issue we don't understand : we are not able to change the status of some IF from packed to shipped :

NT returns the error message : "you can not over-fulfill an item unless you have selected the "allow overage on the item fulfillements' preference"

This preference is false in our configuration as we don't want to be able to fulfill several an item.

But i still don't understand how we have been able to fulfill more than once some item of the IF

i don't understand where the problem can come from as an item can be fulfilled only if available and fulfilled only once,