Finish button twice on Checkout

-- HikaShop version -- : 2.6.4
-- Joomla version -- : 3.6.2

We are experiencing the "click Finish twice at checkout" problem but in a weird way.

First both Auto Select and Auto Submit are set to yes. Second, we have no custom views for payment or shipping methods. Thirdly, the problem only happens under THIS pattern:

add any product to cart as a guest
Login AT checkout
Select Purchase Order for payment method "Hikashop Purchase Order payment plugin" - requires special level ACL
Enter PO number
Accept terms
Click Finish
Page reloads exactly as before
Click Finish again and order confirmation is submitted.

If the user is logged in before entering checkout the problem doesn't happen.
I see no errors nor anything in the logs