I can confirm this on MI 9T. The problem is present when AOD is off but works fine when AOD is on even if you set AOD to be on for 1 minute a day.I'm also having this issue on the Mi 9 SE. Can barely see the fingerprint unlock icon at all, even on a dark area. I can't recall if I had this issue in the previous week build, if I did have, I didn't notice it.
The fingerprint icon is barely visible all the time and doesn't adjust itself, according to the the light available at the given moment, I believe it's the way it works, even if you have auto brightness off.