Everytime I build the lighting, everything comes out wrong with foliage. for some reason they go either super bright or extremely dark. Thanks in advance 
Not sure if you ever figured this out or not because I’m still dealing with this issue in UE4. its frustrating to say the least.
I found it, foliage actors don’t support built lighting for LOD’s, so you either need to remove your LOD’s or not build your lighting. I’d go with removing foliage LOD’s