I had a similar problem with a level that failed to port probably from classic to DE. It may not be entirely the same case, but I have a few ideas you could try.
Create a new level as a copy of that level, see if the terrain works then. Or mark the terrain and duplicate it by holding shift as you drag it up, then just change the coordinate value to be precisely where the old terrain was. Then regenerate AI. I can't fully recall if that's what solved it for me or not, but I can't immediately think of anything else, so it's my starter ideas anyway.
