have the same trouble with a 8x8 1009 tileset.
reimporting via world composition does nothing
reimporting via landscape editor tile by tile works but is awful and i get a unshaded lod0 if i reimport more then 2 layers and a crash if i load more then 3 layers
im on 4.16.2
my weightmap pngs are located outside of project root and always stay in the same place on my local machine