You have to disable fullscreen gestures. I had the same problem with boon. Go to Setting type in the searchbox "Fullscreen display" and switch from full screen gestures to Buttons
Since MIUI 12 some apps don´t work (security reasons) because display overlay is detected. I deactivated display overlay for all apps except system ui because I can´t deactivate it. Anyone else with this problem?
If MIUI 12 will be released, how should I install without loosing Data and Apps? First wipe System and Dalvik and then install new image? Sorry I´m a absolute newbie