Hi, Try to disable and re-enable it. You can do this under Face unlock settings of your phone. When you disable the feature, make sure you restart your phone before enabling it again. If that doesn't work try to delete your face data and register it again.
Most welcome, if the issue still persist after the last two steps the try to wipe catch partitions; Go to Recovery Mode and WipeCatchPartitions, switch off your device, then press Volume down key + Bixby key + Power button until the starts up and galaxy logo appears, scroll down using Volume button to highlight Wipe Catch Partitions and select it using the power button, then restart your phone.
