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If the loan was paid in full and no portion of the loan was IIB, then it is indeed inaccurate and you need to contact them in writing if they are verifying the disputes.
Your mother should have payment records and you can request an investigation and payment history to support their claim of IIB.
Don't take it personally, everything is computerized and if you are correct, you can get it fixed with some time, patience and communication. USAA is a good organization.