Import mapping of customer reference to customer order number not working - b2b sales order syncing

I have BigCommerce B2B sandbox, customer reference number is available in BC checkout as a optional field. P.O and CC are the payment option.

When selecting P.O as payment, everything is good, P.O number is imported to its right field in AC which is Customer Order Number. 

So here is the issue, I want to make use of optional field, customer reference number, and once a customer select a CC as a payment option, that field should be in Customer Order Number field in AC. So we did mapping, Customer Message to Customer Order, however AC seems to ignore the mapping on that field, its just weird that I map same customer message to different field in AC (like description) and its working fine..