I am seeing this on Safari Version 14.0.3 running on Mac OS (11.2.1) Open two instances of Safari ( let's call them A and B ) side by side. On A) open https://makarandp0.github.io/twilio-video-api/ and click on "+ Local Video Track" This simply calls GetUserMedia to acquire a video track and renders it in a Video Element. It also shows state of the track. On B) repeat the same step on B, notice that video track on A gets muted and turns black.
This is by design: only one page at a time can get camera and/or microphone. The user can manage this through the camera/mic red icon. The web page can rerequest getUserMedia access, for instance when it gets its tracks in muted state. getUserMedia will then only process the request when the page is visible.
<rdar://problem/75700376>