I don't know what payment terms you have, but normally if a guest changes his or her mind about staying at your place they would not be refunded or at least only refunded within the cancellation terms you set up. Unless you decided to voluntary refund them more than they were owed under your cancellation.
It sounds as if you were paid for the guests stay who cancelled, which BNB subsequently refunded so they are now taking the payments out of your future bookings.
Perhaps give them a call to talk through if you want them to talk you through it.