This does not happen in the 2D Editor could still be a bug with the mod however highly doubt it. Have asked the mod author and he says he has experienced same problem and it is an eden editor bug.
Sorry if it isn't. I have attached a link to the zombies and demons mod just in case.
https://www.dropbox.com/s/bedsg3xdpoqqted/Zombies%20%26%20Demons%20V2.8.zip?dl=0