I believe this is an issue with the client, not the server\realm itself... I think this happens on all private vanilla servers.
See all the top results here, numerous different servers:
https://www.google.co.uk/search?q=falling+through+zep+mac&pws=0&gl=uk&gws_rd=cr
The way around it is to play via Wine, or at least switch to a Wine based client to travel between continents, not ideal, but it's the only way around it I've found.