[Solved] Xiaomi Redmi Note 8 Pro and 9 Pro: Disrupted Messenger Notifications Post MIUI 12 Update

Hello.
I have 2 Xiaomi Redmi Note 8 Pro phones and one 9 Pro.
I noticed that after updating to MIUI 12 Messenger notifications stopped coming.
Someone sends me a message and I'm deaf. When I accidentally click on Messenger, I have the impression that it has just started and shows that something has arrived. But if I don't check for a day or two, there are no notifications. I have the impression that this application shuts itself down instead of running in the background.
Anyone have any idea what to do with this kid?

Power saving tends to be aggressive in MIUI.
Settings->Battery and performance->Cog in the upper right corner->App saving.
Find Messenger, Power saver->Unlimited.

I managed. It turned out that you had to go to settings-applications-application management and select messenger and turn on "autostart". Notifications are back.
If any other application does not give notifications, you need to do the same with it.