I noticed a problem where if you enter and exit playmode rapidly before the tiles are done generating, it will soon break the graph. I am doing this to the Desert dunes graph that comes as one of the examples. It went from looking like:
To like:
Which the main difference is the previews, which isn't bad since it still generates the terrain looking as it should. But, I do have it waiting on the onAllComplete, and noticed when this happens, the tiles never complete loading. I only assume this happens if you spam playmode on/off since that is when it happened to me. Whenever it happens, the only way i can make the onAllComplete action work is if I set the main and draft tile ranges both to 1. Specifically to 1 too, if I have either not be 1, it will never trigger the onAllComplete and all tiles will never finish loading.
So if you want to try and replicate this, make the main range to 1 and the draft range to 2. Then, just spam the play button on and off until the preview images start to mess up. You should also see the loading icons above the tiles in scene view stay there forever. Also, try subscribing something to the TerrainTile.onAllComplete to see if it works. I also have my terrain resolutions to be 513 for the main and 65 for the drafts, that seems to effect it as well, where if it's not those it will not finish loading.
Also, I am using the 2.0.4 version with all modules installed and updated to 2.0.4.