PO Status error. It changed automatically

Step how i get PO status changed?

(1) Open Purchase Order where status Completed (GRN fully received)

(2) Change status from Completed to Open Order and save record.

(3) Without change anything, click save record again (Mean, user click button save two times already). When user click save for second time, status will change from Open Order to Completed.

(4) After that, press refresh button and status will change back from Completed to Open Order.

Any idea about this?