Application overlay impossible Miu

Hello,

For many months now (and it's getting worse), it has become impossible for me to have multiple applications running at the same time on my Android Redmi S2 phone.

All applications are "killed" by the system as soon as I put them in the background (it's impossible to listen to music and browse Chrome at the same time, or to check my calendar)
It's impossible to have 2 applications running simultaneously!!
It's very limiting because now even having my keyboard and Chrome (or another app) doesn't work well! So, it's hard to write messages.

I've tried (in vain):
Putting the lock so that apps stay in the background

Removing battery restrictions from applications

Resetting the phone to factory settings (the problem was solved for a few minutes but then it quickly returned)

Checking the webview application and clearing its cache

I've tried many other things but I can't remember them all, and they all ended in failure.