Xiaomi
Lock Background Apps auf Xiaomi
On MIUI, background apps are managed by an aggressive battery optimizer that closes processes it considers inactive. This is why messengers stop receiving notifications or a running timer resets after you switch to another app. Locking an app tells the system to keep it alive instead of freeing its memory.
On older MIUI versions this lock could be set directly from the recent apps overview by tapping an app's icon. On several current Xiaomi devices that shortcut is gone. The lock function moved into the Security app, under the speed optimization tools.
How to lock an app
Open Security ➔ Settings ➔ Boost speed ➔ Lock apps. This list shows every installed app with a toggle next to it. Turn the toggle on for any app you want to keep running in the background, or turn it off to let the system manage it normally again using Turn On/Off an App.
Locking too many apps at once increases battery drain, since none of them get closed automatically anymore. Lock only the apps that actually need to stay active, such as chat tools, VPNs, or fitness trackers. For everything else, leave the toggle off and let MIUI manage memory on its own.