Re: stat. del. dte. past than the document dte of PO

do some tests in your QA system.

create a PR like yours, create a PO from it without changing anything, then check DD and SDD.

create a PR, create PO and change the SDD to a last years date while creating, Save and check change history, DD and SDD

create a PR, create a PO, do not print, change the DD, save, check DD and SDD.

create a PR, create PO, print, change DD, save and check DD and SDD.

After these tests you know how your SAP works,

you know  if the dates are like you entered them or if they somehow look different and manipulated (e.g. via user exit)

and you know if this case would be caused by SAP or more likely by a user error.