PO using Approval Framework improperly advancing to "Approved" by making changes on the Distribution

We've identified a hole in the PO Approval Framework that's allowing a PO to advance to "approved" prematurely and I'm looking for some advice on a possible fix.  This involves an adhoc PO that's initially created in "open" status.  When the PO has been saved and a PO ID assigned, at this point the approval path looks normal.  But before submitting the PO for approval, the user goes to the Distribution page to make a change (in this case the Inventory BU field) and saves it.  When the user then returns to the main page to submit the PO and clicks the "submit" checkbox, the