Unfortunately, it's really up to the discretion of the app developer to decide what "back" does. There are guidelines and "best practices", but nothing dictated

Unfortunately, it's really up to the discretion of the app developer to decide what "back" does. There are guidelines and "best practices", but nothing dictated by the API or operating system. The back button always goes backwards in the history stack, but the app developer can push new pages onto the history stack however they see fit. An analogy on the web is either navigating to a new page (back button works), or loading a new page on top of the current page (back button doesn't work).