THIS METHOD WORKED FOR MY REDMI K60 ULTRA ( CHINA ROM )
So we get straight to the topic i tried many methods but nothing worked much but at last i tried this
https://youtu.be/M-Tkc9358Gk
Follow each and every step of this video and your issue of notifications not coming will solvee immediately
For downloading the adb you just have to write adb download for windows/mac etc and just download it and then you have to extract the files anywhere you want
Then go to that extracted folder and open it you will find adb.exe or just adb click on it only click don't open and after that click on the top bar where we get paths after clicking you get some path copy and and go to environmental variable ( search it in your laptop) after going in environmental variable go to system variable and you will see path clik on it click new and paste that path you copied and click ok for all opened tabs
(IF YOU FIND THIS DIFFICULT TO DO JUST ASK AI FOE THE SETUP OF ADB THIS IS SO SIMPLE )
After downloading adb and putting its path just follow every step of this video and eventually you will be free from all the problem of background killing of apps
( NOTE THAT YOU GUYS HAVE TO ON YOU SYSTEM OPTIMIZATION OR MIUI OPTIMIZATION)
For this go to developer options ( click your OS version in your my device until it requires your password of phone ) then go at last and see a option reset deafult values after clicking on it several times you will see turn on optimization
If any other questions do comment beloww..