I've also got this problem. makes it difficult to add new materials XD You can add them manually by editing the main.materials.json file in the terrains folder, but that's definitely quite tedious.
That's the point I'm at. The tedious part is trying to figure out the size for each texture file (diffuse, macro, detail). Can you edit the json while the level is active and have it update in real time? I've been exiting the level for every edit.