Right now, Lumen really doesn´t like complex shapes, or to thin walls.
Have you tried to simply rebuild the walls from simple cubes in that region? Then you could make them thicker, to check, if the light gets blocked.
While i was unable to recreate your problem, at least one sideeffect of to thin walls was, that the light started shining through gaps, where two thin walls connect, that still looked like one massive wall.
One noticeable effect of complex meshes is, that several parts won´t work with Lumen and will be ignored for f.e. it´s global illumination (in such cases, it will fall back to screen space stuff, if i understand that correct).
I tried it with a test labyrinth, one made from one big complex mesh (left side, with all those pink and therefore unsupported parts, which Lumen will ignore), and then i rebuilt the whole labyrinth from just cubes, where every wall is a separate segment. In terms of global illumination, the differences are quite noticeable in areas, where the light has to bounce often, to illuminate the place.
Check out those dark corners in the closeup. Both are taken from the same spot in both labyrinths, top left corner, where the top left glowing block sits. Left closeup is pitch black, while the right close up from the labyrinth built from separate blocks gets illuminated better.
Note: On the right side, those small pink areas vanish, once i get closer. Same for the yellow blocks, guess thats some distance culling. The left side always stays a pink mess, no matter, how close i am ^.^