This reminds me of the scenarios people came up with when FaceID was first announced:
“A person could just grab my phone out of my hands, shove it in my face, then run away with it!”
If the person was smart, they would wait until you unlocked it yourself before grabbing it, regardless of which security method you’re using.
Plus you’re assuming that the person is able to grab the phone, wake the screen, make sure it’s at a correct distance, give it a second to scan your face, and verify that it unlocked all without you reacting in any way.
This is an interesting bypass, and a good way to challenge Apple to improve FaceID imo. But the real-world practicality is very low.