To Fix the issue go to settings > battery optimization > click to view all > and disable battery optimization for com.samsung.biometrics
Sad that samsung can't fix this themselves
Thanks a ton for this solution! I was so frustrated with my new S21 and face unlock not working. This fixed my issue, but the name of the com. value seems to have changed on the s21. Here is what I changed:
Settings menu > search for optimize > select optimize battery usage > change list to all.
turn phone to landscape so you can see the full names
look for com.samsung.android.biometrics.app.setting
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