It definitely does not go back to the card it was purchased on.  If you purchase OBC (refundable) ahead of the cruise, it is applied as a credit on your onboard account (hotel folio) at the start of the cruise. When you check in for your cruise you give them the card you want to use for any onboard purchases. At the end of the cruise, your onboard account (hotel folio) is settled against the card you used to check in.    This is all very similar to how hotels work where I might give Card
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