When you go to save your map, create a new folder in levels and save it into that folder. Edit your terrain to your liking and save your map. Once that is done, you need to copy your terrain file to a new folder and rereference it if you plan to share the map. It will be located in "BeamNG-DRIVE-0.3\art\terrains" and have your terrain's name that you gave it. Copy it to "BeamNG-DRIVE-0.3\levels\<Your level name>\art\terrains\" When ingame, select your terrain object within the editor, you have an option to select what .ter file you want to use. Browse to your map's folder, into the art\terrains\ and select your terrain file. Now when you save your map, it should always keep your terrain. Hopefully this clears up this issue for a few people. tl;dr .ter files save to <BeamNG Root>\art\terrains\ but you need to put them in your level's folder and reference from there instead to share it.
