On any save file, any terrain, and any world, the off-the-grid terrain challenge activates on its own constantly, and approximately every 2 to 5 minutes.
I constantly have to go into the build mod, enable it (because even if the terrain is marked as off-grid, the terrain challenge doesn't actually appear), go into live mode, go back into the build mod, and disable it.
It's very annoying to play like this, and I haven't seen anyone else experiencing this bug!