Alarm (clock) Does Not Wake Up...

Hello.

I am using xiaomi.eu stable 8.1.8.0.

I have a very annoying issue : alarm does not wake the phone up !!
To be more specific, alarm works if I unlock manually my phone when it is supposed to ring, otherwise phone remains asleep.

This issue appears only after the whole night in standby. If I set it up 5 minutes from now, it will work properly.

Is it a standby/deepsleep issue ? Any fix please ?

Thanks. Regards.