okay. well… i noticed a certain directionality of the issue. i dunno how the tangent basis in unreal looks and works. but… i replaced all the meshes with simplifcations (without the bevels) and it fixed it.
to verify… i exported and reimported all 5 meshes with recomputed normals and it fixed it, partially, too. it now generates artefacts on the floor where the holes are in the walls.
and… well… all those holes have bevels on it. distance fields should be fine dealing with micro geometry but i reckon lumen meshcards are generated with some form of faulty directional data producing the artefacts. i dunno howto debug view this, but it’s an educated guess.
could be the same cause for the double sided material issue. it possible it traces in random faulty directions and potentially thru geometry, showing as random light leaks.
that’s all i can observe here. for proper technical debug you may have to wait for staff or a more advanced graphics dev to pick it up.