We have noticed that many users using Xaomi Phone have had issues with notifications. This is because Xaomi uses some very aggressive battery saving measure that can block apps from sending notifications.
Please follow the steps below to fix the issue, if they still don't work after that, drop me an email at firstname.lastname@example.org
1: Go to settings --> permissions --> autostart. From there, pick the apps you want to receive notifications, and toggle the switch to turn it on.
2: Go to settings --> app notifications --> pick the apps you want to receive notifications, then turn the toggle for "Priority" on.
3: Lock the app in the "recent apps"/ "app overview" plane. Do so by first opening the apps, then press the "recent apps/overview button" (that's the square button on stock Android, or the button with three horizontal lines on the Mi Mix). From there, find the app you want to receive notifications, pull down on it to "lock it", so it never gets cleared.
4: This last step requires Developer Options privileges. To enable that, go to settings --> about phone --> tap on MIUI version tab like eight times. You should then get a little message saying "you are now a developer." Then head back out to settings, go to developer option, scroll to nearly the bottom, find "memory optimization," and turn it off.