i_am_sy you'll find many apps certainly on Android work the same way, so it's consistent with them.
With respect I've been an Android user since Honeycomb and I've rarely seen a back button behave so erratically. In fact I'm struggling to think of even one app, ever, where a Back button doesn't actually take you Back. 🤣
If this was common behaviour then nobody would be complaining.
If the developers want this button to have some other functionality, such as moving up a level, or taking you to the top level from your current location, then they need to redesign the icon and actually make it behave in a consistent manner.
Currently it behaves as a Back button sometimes, depending on where you are, and at other times it moves you to top level.
There's no greater sin, when creating a UI, than confusing your users with inconsistent behaviour.