Why does the Xiaomi Smart Band 8 display disconnected when connecting to the phone?

It is possible that the background of the Mi Fitness APP has been cleared, so please confirm if all background running permissions for the app have been enabled.
There are two paths for setting:
1. After the first connection between the device and the app, tap to enter the settings option;
2. After pairing, go to the [Mi Fitness] APP >> [Background permissions] >> [Select “No restrictions” in Background settings].