You must explictly declare which banks you want to preload in map properties. There's option for it somewhere in menu, should be called "Preload info".
In case of using Trigger GUI elements, the editor does that for you (at least it used to), but in case of raw script it doesn't, as it doesn't process it.
btw. Small notice in regards to your code, when bank fails to verify it doesn't mean the data from it hasn't been loaded, but only that the signature check failed.
And calling BankLoad again won't erase the data here (unless you do that manually by iterating over sections). Thus you likely want to call BankRemove after BankVerify fails. This guarantees it will be cleared.