Velu12 in this screenshoot
are the mesh overlapping?
Also, to test if the is the occlusion culling you can re-create the same level/rooms with only the Unreal BSP with OC on. Or you can use the same room, but add space between all mesh, just to test.
I think this is a overlapping mesh, cause looks the problem only happen where have a 2+ mesh intersection.
Let us know 