To reproduce:

1. Open attached project (754298.zip)
2. Open "terrain_shadow" scene
3. Open Lighting window
4. In the Scene tab, click Build
5. Inspect scene view
6. Switch Visualization mode to Clustering
7. See that clustering mesh has different triangulation than the original and is overlapping the planes

Expected behavior: right shadows on all four quads
Actual behavior: two quads got no shadow, the other two got wrong shadow (see screenshots)

Still present in 2017.2.0b2