By allowing the account to be file maintenanced into a different form of ownership, your bank may have bought itself a law suit.
It has created an ambiguity which simply would not have existed had the individual's personal account been closed as it should have been when the guardian was appointed.
It's highly unlikely that the original POD beneficiary will be made aware of the situation, but if he is he will probably sue you for the funds. Rest assured, I would.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.