When you are using webcam and screen share simultaneously, it's important to remember that your webcam will remain active unless you manually turn it off. For instance, if you were speaking to your team through your webcam and then clicked the Screenshare button in order to share a file on your screen, your webcam would still be active behind the scenes. Once you stopped sharing your screen, your webcam feed would be displayed in-world.
For seamless transitioning between webcam and screen share, first make sure that you've turned on your webcam and affirmed that Breakroom has permission to access your camera. Then use the Screenshare button to toggle screen share as needed, keeping in mind that your camera will remain active until you turn it off.