Hi,
After a day or two with FOD disabled on my Mi 9, it seems in fact that deep sleeps works way better and keeps steady throughout the day.
Strangely enough, there's that last option in the fingerprint setup menu which, if enabled, allows the digital fingerprint icon to be on when the screen is off. It is stated there that if it is enabled, the battery consumption will be higher. However, I've tried some time with it off and it made no difference (meaning, deep sleep went down to 0% after a few hours).
So, although I will continue to test battery consumption/deep sleep with FOD disabled, it seems in fact it is the root cause of this issue.