After I updated to 8.12.27 the fingerprint unlock doesn't work, I noticed that from settings the option unlock with fingerprint gesture isn't available anymore.
Edit: Solved, I reboot again and it's fixed.
I have very strange problem with my Redmi Note 4 MTK.
Sometimes I cann't use my device. When I'm touching screen phone is getting locked (black screen) and asking me again for password/fingerprint (like after double click on PWR button).
I made update of MIUI, wipe data and it helps...