Title bar in ME29N is of the format “Release &1 &2” where &1 is the description of the PO Document type and &2 is the PO number.
Title bar in ME22N/ME23N is of the format “&1 &2 Created by &3” where &1 is the description of the PO Document type, &2 is the PO Number and &3 is the User Name ( Fname+Lname)
If you click on the change-display icon in ME29N it will take you to the PO in display mode and the Title bar shows the User name because it takes the control to that of ME23N title display.
Please note that these things can not be changed as they are not controlled by any configurations. These are standard SAP features and can not be changed without developments.