No, unless there is a valid and joint decision made under the POA.
All acts/decisions your mother previously made, including access to her account, remain valid acts.
Imagine telling a bank that she no longer has to pay the mortgage she signed because we now have POA. Or you telling the bank to transfer all your mother's money into your account just on your say, without your brother's permission.
I haven't checked timelines to know if you can still complain about the Bank's response. Clearly they avoided answering the question being asked. I'd go back to the FOS and complain about the Bank's failure to answer the question.