One of the issues people lucky enough to have Pie are noticing has to do with the back button in the new gesture navigation setup.
Here’s what happens: you open up an application (like Google Play Music, for example) and swipe around that app for a bit. Then, you decide you want to switch to another app you have open, so you swipe up on the “pill” icon to see your other open apps.
However, after opening the recent apps you realize you don’t want to switch apps, and instead want to go back into Google Play Music. Naturally, you hit the back button.
For right now, anyway, the back button would not take you back into Google Play Music; instead, you get taken to the home screen.
This seems like a little thing, but it could get pretty annoying. Intuitively, the back button should take you out of the recent apps and back into whatever it is you were doing when you swiped up into the recent apps switcher. If that was a home screen, then it would make sense to go there. But in the example above, it should take you back into Google Play Music, not the home screen.
Back in July, a user brought up this issue with the Google team. Yesterday, the issue got marked as “Fixed” and a future patch to Android 9.0 Pie should rectify the situation.