Dear Asus team,
I ran into problem when using split screen function as I can only use navigation back button to control the secondary app. I can not use the navigation back button to use on primary app (as in the app displayed on top of the screen). I tried tapping the primary app first then using the "back" button, but it still ended up controlling the secondary app (at the bottom on the screen). My phone is running the latest firmware as of today.