You accidentally briefly swipe left while unlocking, causing it to flash the camera, which causes the camera indicator to stay on for a while. You can reproduce it by locking your phone, touching the screen once, then swiping a centimeter left then back.

No guarantees—I’m no expert: Turning off Face ID doesn't mean turning off the camera. Under "Camera," you should be able to see which app used it last. Settings > Accessibility > Face ID & Attention > Attention Aware Features and Settings > Face ID & Passcode > Require Attention for Face ID. Turn it off and restart.