Battery drain issue on idle phone, how to find out the culprit?
I had this same issue, it seems to be related to media control apps like Spotify and Google Home mini always staying on and keeping wifi awake.
Also check for any other network connected devices that could be spamming. Could be a printer, or a laptop even, anything on the network is a suspect.

Also try this which seems to have helped:


Removing fingerprint unlock. Perhaps I had 3 fingerprints, before I had 2, maybe that affected battery life and device being active.
Removing one of the themes from the miui Themes app, it seems dodgy, using default theme seems to be helping too.


Network changes:


Some routers and your phone's wifi chip might be a bit funny, so perhaps switch to Static fixed IP address for your phone instead of a dynamic DHCP ip address.
Also try fixing the Wifi Router's Channel to a specific channel instead of leaving it on automatic. (Use WirelessNetView on PC to see which channels are quieter in your neighbourhood, choose between 1, 6 or 11 only)