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Strange dark artifacts on imported mesh

Whenever I import a mesh, it creates really bad dark-colored artifacts on it. Even when exporting a perfectly fine mesh from the editor and re-importing it, these same artifacts will appear. Opening up the mesh, the preview window showing the mesh has black spots all over it. in the level, these spots are light brown, and the shadow of the mesh is just completely mangled and broken. I’ve searched for a while and cannot find any solution to this problem that has worked in my case.

The program I am using for modeling and exporting the files is Blender 2.8.
I have exported the mesh from Blender and into UE4 multiple times under both the OBJ and FBX Formats.
I do not have any materials applied to the model, besides the default checkerboard tile material.
I am using Distance field shadows, and completely dynamic lighting (No static baked lights).
I have checked and recalculated the normals multiple times, to no avail.
I have also made sure I am using flat shading, and exporting the normals correctly.

Other meshes seem to import fine (I exported multiple models from Portal and put them into UE4 and all of them look normal), But it is just this cube that will not import correctly.
The reason I am importing this cube instead of just using the regular cube mesh is because the one I am importing has different material slots for the top and bottom than for the sides.

You need to properly set up lightmap UV’s

How do I export the lightmap UVs from Blender? It seems that UE4 generates new lightmap UVs for the mesh when I import the mesh.
(Side note, I want to use the FBX format because it has the right scale for everything else I’ve made)

Edit: Nevermind, I have figured out how to do this. The problem remains though. despite having the custom lightmaps set up and applied.