I was able to reproduce the behavior on your demo site, as well as on our site. It happens when you just type in text. You can try it yourself - Just type in some random text, maybe a little bit faster (not necessary), and you will see, that some of the characters are missing.
Dunno where this is coming from. Maybe some JS/jQuery-problem with the editor script? I checked it on Chrome, Firefox and Vivaldi, but didn't had the problem. Only on IE/Edge.
Best regards!