I have found out it is because not all comms.send unfocus gets received.
I solved most of the issues I had with it by ensuring that the reader page is not navigated away from until Navigator.destroy() has finished, however it still sometimes hangs. I suspect it is if I navigate fast in a publication that it is causing the problems, as that is how I can recreate the problem with nav.destroy hanging and when this happens I get event for _ping was never handled in the console.