We use 10.0.7.4 and have been live since last year and yes PO suggestions will drive you nuts. I do know when we spoke to our consultant he mentioned that many flags influence this - if they are not filled in the system will try and do lots of clever but infuriating stuff for you - I think in this case you have a MOH which you are falling below, the system is generating a suggestion but at the same time is then working out when your next demand is and telling you that you don’t need to buy that much material at this time.
Locking the PO does help but if we also populate the resource in and resource out delta’s in part | site | planning with a number of days it basically does nothing with existing po’s if they fall within that date range - it will still make suggestions if you are short/need material in that range but if you have a manual po that meets or exceeds demand it will not tell you to fix - I think these fields exist in version 9