Sure they might find out an App Update was issued in the App Store... but they cannot possibly know what API changes were made without reverse-engineering the API's all over again.

Not to mention, once they discover what changed, now this service has to go make the changes on their system... meanwhile that bank doesn't work with this service anymore.

So the App is broken for some undefined period of time.

It seems like they would only find out after the update has been pushed, though - ie, after the app has been broken.

Unless they have relationships with the banks now, and would be given a heads-up. Perhaps that's the case. It sounds like it may be.