as its a famous problem in mi band 4 that if you moved away from your phone then came back every thing will be working but the incoming call alert, then you need to switch off the bluetooth from your mobile then turn it on again to get the incoming call alert working again.|
from your mobile go to settings lock screen & password advanced settings on lock screen Show all notifications and their contents.then remove the band from unlock with blutooth device section.
now its working great and you can move away from your mobile and when you back again the incoming call alert working great :D
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