NVM, I found the problem in https://github.com/driftingruby/042-fullcalendar.
In app/views/events/create.js.erb and app/views/events/update.js.erb
$('.calendar').fullCalendar(
'renderEvent',
$.parseJSON("<%=j render(@event, format: :json).html_safe %>"),
true // <-----
);
the 'stick' atttibute for 'renderEvent' method should be set to false