Hi
This is standard behavior.Once the order is technically completed the PR item are marked for deletion.
There is no exit to prevent this.Using IWO10004 user exit you can put the check or pop up that this PR still open or not converted in PO.
If in case user do TECO by mistake before PO conversion then you can reverse the TECO and automatically deletion flag will be removed from item and then convert it to PO.
Thanks
Regards
Gaurav Solanki