Update known-issues.md

Author: enricohuang

articles/communication-services/concepts/known-issues.md

@@ -61,6 +61,10 @@ A number of specific Android devices fail to start, accept calls, and meetings.
 
 On Android Chrome, if a user is on an ACS call and puts the browser into background for one minute. The microphone will lose access and the other participants in the call won't hear the audio from the user. Once the user brings the browser to foreground, microphone will be available again. Related chromium bugs [here](https://bugs.chromium.org/p/chromium/issues/detail?id=1027446) and [here](https://bugs.chromium.org/p/webrtc/issues/detail?id=10940)
 
+### The user has dropped the call but is still on the participant list.
+
+The problem can occur if a mobile user leaves the ACS group call without properly hangup. When a user closes the browser or refreshs the webpage without hangup, other participants in the group call will still see the user on the participant list for about 2 minutes.
+
 ### iOS Safari refreshes the page if the user goes to another app and returns back to the browser
 
 The problem can occur if a user in an ACS call with iOS Safari, and switches to other app for a while. After the user returns back to the browser,