Hi again :-),
and don't worry, you were on holidays! those are very important, too ;-).
I guess you are facing the issues because of all those Workflows interacting at some point together...
Maybe you expect the results in just 1 workflow, but the conditions of other different one applies at certain point and other responses are executed that makes your PO to behave in a different way and come back to Open Status...
The strange thing is when you have the PO Approved, it should have Released status.
Then your people should post it only to get the Invoice, right? I don't get the point that it goes back to Open status to be able to convert it to Invoice, sorry.
Again :-), if you are not able to identify the real issue, or if you need extra help with that, we are willing to get your support ticket in Microsoft Support ;-).
Kindest regards and enjoy the day!,
Vanessa.