Hi,
Please check whether any confirmation / inbound delivery exists for the PO. In this case, these dates will be displayed as delivery date in MIGO. Refer the KBA: 2010901 - Delivery Date is not Displayed in transaction MIGO which explains the standard logic.
You may also remove the "propose all items" flag in MIGO - menu - settings - default values, which will avoid future delivery schedules being proposing in MIGO.
Regards,
AKPT