In general, all new Android versions
Google has made it impossible to launch screens from the background in newer Android versions. Because of this, we can no longer show the alarm screen with the big buttons in the usual settings. The app can only show notifications in the background if the screen is on when the alarm starts. If the screen is locked, we can usually show the full screen alarm.
This is a companion discussion topic for the original entry at https://sleep.urbandroid.org/docs/faqs/alarm_lockscreen_xiaomi.html