To avoid this in MIUI, please do few system settings.
1) Open Security App --> Battery --> App Battery Saver. Now, tap on Mubble, then ""No Restrictions"".
2) Security app again --> Permissions --> Permissions --> enable autostart for Mubble.
3) Again, Security App --> Permissions --> Other Permissions --> enable Service Message for Mubble.
4) Lock mubble in recent task list.
If it because of some task killers and battery optimizer, add Mubble app to the White List of your Task Killer , so that it doesn't turn off Accessibility for >>Mubble<<. Also lock mubble in recent task list.