This seems like a long standing issue. Reproed from M53.
Closing tab when casting to Chromecast device will trigger onEnded event for video track, and MR will stop audio track when receiving this event.
For audio only mirroring, when the tab is closed, no onEnded event is triggered, and session can't be stopped.
Comment 1 by sko...@chromium.org
, Oct 14 2016