Currently, with the new accessibility settings introduced, the setting to change my active webcam is not available and defaults to the first on the list (which is my capture card that defaults to black when nothing shows)
I can’t reproduce this 100% of the time; on my mobile device (iPad Mini 6) with accessibility settings rolled out, the option shows up at the bottom as Video Camera. However on desktop the option is completely absent.
Mobile:
I have found a workaround! It appears that it only disables changing the camera in the settings with the new topbar (That you can set using the "FFlagEnableInGameMenuChrome": True flag). The only workaround is to disable that flag, change the camera, and turn the flag back on after doing so.
People who have the new topbar permanently cannot use this workaround. I am sorry!