Hi, according to some Xiaomi forums your issue could be related to a kind of aggressive battery saving that MIUI uses.

Please try the following:

  • go to Xiaomi security app → Permissions → Other permissions → Avast Mobile Security → Display pop-up window → Accept
  • go to Xiaomi security app → Permissions → Other permissions → Avast Mobile Security → make sure it is allowed to run

You can also try to follow instructions from this article:
https://www.androidcentral.com/how-fix-push-notifications-miui-8