I think the maps have blacklisted since they're behaving the same way the rest of the original, unmodified 1.13 maps do. If I change the map file name to something other than "LonelyCave" it starts working but changing it back brings up the connection problem message again. I'll upload a fixed version and update the first post soon... ish.
Edit:
Thank you so much for this! However, I am having a bit of trouble on mac. I can't seem to find a root file at all, so there's no way for me to move the maps. Any suggestions?
Watch the tutorial video on my first post, the root folder is the folder where Feral Heart is located not the root of your whole computer. If you installed the game the way Raz says to
here all you need to do is open PlayOnMac and browse the game's directory and drag/drop the files into it.