I think, if you convert 2 objects to a new static mesh, unreal will keep the materials. Imagine you got a box and a glass door and want to combine them. of course u need the glass material, and the wooden material. In your combined mesh.
However, if all of your assets uses the same material, then the “Merge Actors” or BSP “create static mesh” will only assign 1 material on the whole mesh, even if it’s a material instance.
So i created a some BSP-Cubes, set material, converted them to SM, only 1 Material slot used, I Merged 3 static meshes with the same material, only 1 slot used. So how exactly do you merge the meshes? do you export as fbx or something?