Multiple mistakes along the way can lead to balance billing or fraudulent billing. If Medicare asked for clarifying information and MBD did not follow through then that turns into balance billing (or fraudulent billing if they knowingly closed the case without providing the information).
Balance billing is trying to get money from the patient over and above the negotiated contract. The MBD is not allowed to balance bill you or close your case until all issues have been resolved. If Medicare says “Patient responsibility” is zero then that is part of the negotiated contract. If MBD had provided the clarifying information then Medicare would pay their portion and let you and them know your responsible portion and then that responsible portion goes on to your supplemental insurance.
Multiple mistakes along the way does not relieve the billing department from fraudulent billing.
If Medicare says $0 at this time then any amount they try to come to you for is balance billing and balance billing is illegal.
Insurance companies can also deny coverage for not billing in a timely manner but that is the responsibility of MBD and not the patient.