I have enabled Trusted face, also it comes with pattern lock if face is not recognized.
But it has a flaw, anyone can unlock the phone with camera/phone call shortcut on lock screen. As shown in attached screenshot, is it possible to prevent this?
I tried what you're saying a few times, and after you close the camera, you're back on the lock screen.
Unless you were actually looking at the phone, it recognized your face and it unlocked itself milliseconds before opening the camera.
You can notice that the "symbol" in the middle goes from that of a person to that of an unlocked lock when it recognizes you
Just as Kushtrim mentioned, I also suspect that it's due to "Trusted face": maybe it's too sensitive, or the face you provided is.. still too general that everyone's face is detected as yours.
Some suggestions that I could think of:
Improve the "Trusted face" recognition. It might be a hit-or-miss, but it's worth a try: