Well if all else fails it’s time to divided the trouble shooting down to find where the problem might be.
You seem to have covered most but for completeness.
It could be in the lighting.
It could be in the source chain from Blender imported into UE4
It could be in the material set up.
The bast practice as to trouble shooting is to split it in half as to what you know should work and what you might suspect is the problem,
In the same project start up a new blank map with the basics. Drop your building that’s causing the problem into the center and added a default material or one you know works and then build the lighting.
If it’s messed up then you know it’s in the source chain.
Next add your material and rebuild.
If it’s messed up then the problem is in the material and to figure out which is causing the problem you can disconnect links one at a time. If you disconnect the normal map, which can cause strange effects, then there is you problem. (note materials don’t always jive as to what you see when applied)
So in a clean room so to speak if it all checks out you just crossed off half of what the problem might be and time to eliminate the other half.
(P.S. If everything checks out you might as well continue to work with the rebuild)
Back in the project map kill off anything that directly effects the light solution. Direct lights, point lights, sky lights, reflection spheres, post process, with the only thing left being light importance.
Note light importance only limits the lighting solution to whats in the box so it saves rendering time of not having to do the entire environment. (ie compile goes from 1 hour to 1 min kind of thing).
Put new fresh single sky or directional light in the environment and build your lighting. If the problem is gone then your light set up was the problem. If not then there is something wrong in the project settings.
The thing of it all is to figure on what side of the fence the problem is originating and could be occurring in the source chain or in the project file so until you figure that out it’s just guessing.
By the what format are you exporting to? I use 3ds Max so I know it all works but Blender has had issues with FBX, which I “heard” was fixed, but their obj exporter I know for a fact does not play nice with other 3d applications.
Last tip don’t worry to much about it. 99.9% of the time if I have some kind of issue I usually land up banging my head on the key board as it turned out to be something simple so your not the only one that’s been there and done that.