Hi,
Please check whether the right purchasing value key is set for the material master - purchasing view, where the "unlimited Over Delivery" field is flagged.
If there is info record, it has the priority. If there is no info record, system reads the "unlimited" flag from the material master purchasing value key and default in the PO. Ensure that the info record field is blank in the PO in your case.
If issue is not resolved, please revert back with screenshots.
Regards,
AKPT