This happens on Windows 10 AFAIK, v6.1.0. Reproduction steps:
  1. Join a call so that the call window is visible.
  2. End the call.
Result: call window is still shown with last image displayed on it, but is completely unresponsive. I have to close the app and reopen it to get rid of it.
Also, after ending a call and starting a new one, this window and its last state (if app isn't restarted) is shown before the call is established and new audio and video comes through, which can be a bit disorienting.