Hi Senzatea ,
We've received some tickets recently related to this same issue you are mentioning and finally discovered that was due to an issue that is causing this when you change the PO status from the POs list (so using the "Mark as received" button or using the bulk actions). Is this your case?
If so, please enter the PO and change the status from there. It should work in this way. In the meantime, we are working on fixing this for the next ATUM version.