To be honest, it is the first time I am seeing this kind of issue with SWARM and I am not sure about what to advise. It appears that you have reached some kind of limit and the map breaks after that.
Have you tried to see if (if you have the latest version) older ones do not present this issue? Or, in the opposite case, if you are using an older version, upgrade to the latest one?
Unfortunately SWARM has been discontinued, so it is hard to get assistance from the original developers.