We have a bunch of foliage in our Map, and some is painted on static mesh and some on a terrain. The foliage painted on the terrain glows with bright primary colours after lighting is built. (the terrain is the water in the screenshots)
Both the foliage on the terrain and on the static mesh are tagged not to cast any static shadows.
"Does the artifact go away when you set the ‘r.Cache.LightingCacheDimension’ console variable to 128? There’s a bug with many (>1728) movable components right now that will cause this. Instanced foliage currently does not support lightmaps correctly which are used when the components have static mobility, so both routes are a bit broken. "
