Yes the Miui Clock is not working in my case, which is already a system app.
Both Miui Clock and Google Clock alarms didn't work this morning.
I already tried the following, but none have worked:
Settings > Battery & Performance > Manage apps battery usage > Off (also I have set apps individually to Off)
Settings > Battery & Performance > Power > Battery Saver > Off
Settings > Permissions > Autostart > Apps have been set to to autostart
Settings > Additional Settings > Developer Options > Memory optimization > Off
Settings > Additional Settings > Developer Options > Miui optimization > Off
There is only other setting I can see that may be affecting this, but I have yet to test this:
Settings > Battery & Performance > Power > (Settings Button) > Clear cache when device is locked.
This was set to 30 minutes. I have now turned it off. I will report back if this makes a difference.
Locking apps in the task list is not a suitable fix as it is still possible to close them by manually swiping.
Also I confirm I have no 3rd party battery saving apps installed. No Greenify, Amplify etc.