I never had deep sleep issues on my Mi 9T Pro until 20.7.9 and i have this phone for 8 months. Couple of days ago I even formated data and clean flashed new beta but it didn't helped.Try prevent Play Store from auto updating and kill Play Store after you run it, and maybe check Auto Start list.
I had idle battery drain on Android 10 version of MIUI 11 Stable, disabling auto update and killing Play Store helped.
I'm trying much stuff to stop it and read this forum every 10 minutes hoping that someone will come with some solution because I really like xiaomi.eu betas and don't want to flash other ROM but I will have to if I don't find solution for deep sleep issue.
Currently I'm downloading Grennify and I will hibernate everything I can and test deep sleep. If that doesn't work I will try your suggestion too. Thanks.