Banks are always ridiculous aren't they? If the principal walks in, I have no clue why they would ask for a GPOA document.
Theoritically, you can revoke the GPOA, register the same, distribute the copy of the revoked document to the bank, and also publish in a newspaper. This will totally cancel anything related to the original POA and shouldn't be in question. If the banks still give you a hard time, you can proceed legally and force the banks to stop being idiots.According to judgments that I have read, the principal can go through a ratification process to address any issues with the GPOA. The courts will totally be on your side if you want to go the legal way.