TLDR -> Set Destination Lightmap Index to 3 on the objects with black faces.
Just started using Ue4 and this was the first problem I’ve encountered, opening the static mesh up I could see that theres an extra UV channel being generated.
UV 0 is a random channel that is being made when imported via data smith.
UV 1 is my diffuse channel made in 3ds Max
UV 2 is the Lightmap channel being made by Unreal.
What looks to be happening is for some reason some objects start using UV 1 as the lightmap channel. So just change the Static Mesh to use “Lightmap Index” 3 and rebake.