That is expected behaviour: fire up a meet now in a channel and lock your machine. If you have a built in camera on a device like a Surface Laptop the camera light will still be on. Audio is still on too.
Would recommend voting up on it. I have voted on it too some time ago. It's really kind of essential moving forward and if you vote you'll get notified if it gets picked up. Until that time users will need to be vigilant and always ensure video and audio are off before locking the machine if they need to move away from it