Yeah once you do something to the watermill map it'll show up on the title screen as the background.
I'm not sure if you can change this. The only thing I know is to re-install Feral Heart and the map will be fine, or come up as it was. If there is another way, I never heard of it. If there isn't, then if you really want the map back and such, remember to back up any important files before installing Feral Heart again.
If there is another way instead of re-installing and someone knows, just ignore this then.
If you have other items you want to keep that did not come with the original Feral Heart, then install it with a different name and only replace Watermill files.