Maybe some help for you
It is well known the when swiping a MCGC, the payment automatically rolls over to a credit, not debit payment. It has been well known in the past that at WM, at least, one could change the credit payment to a debit payment by using an appropriate algorithm that can be found in the past threads with a search.
I have not seen where one can do that switch at SW, but you might ask, or else SW doesn't care whether the payment for a MO is credit or debit (I doubt that but you never know)
I would ask if your debit card (AKA GC) is being used as a debit charge or credit charge. See if you can change the latter to a debit and then try it, if that is possible.
The most recent DPs indicate that even at WM, one is unable to switch MCGCs from credit to debit, but I don't deal with MC so I am unable to add any DPs.