As far, as i know, this can be caused by either the walls being to thin, or by to complex mesh shapes. There was a video, where they break down lumen, and there was mentioned, that Lumen requires simple meshes/shapes, and usually architects built their outer walls in complex shapes → all the outside walls are one single complex mesh, instead broke up in several simple segments.
Take a look at this clip here, at 01:36:20, where they are in that house and switch to the LumenScene View, and most of the interior is black. Maybe your problem is related to this:
