Oh, the undersides of the meshes are being illuminated 
Same issue I had way back here Different meshes that share the same material, batching drawcalls? - #14 by Evgeni_Popov
This was previously solved by passing in a boolean argument to addMesh to indicate right handedness. However since I am using a single merged mesh here, and there is no addMesh call, I may need to indicate this somewhere else?
Seems something regarding the left/right handed system changed in v5
Edit:
using toLeftHanded() on the prefab doesn’t seem to do the trick.