Issue with Lighting on modular blueprint items

Started using unreal relatively recently, total newbie to game design, and I’m trying to make modular room blueprints that I can stack and modify in the level. This is working great EXCEPT the lighting is totally screwy. There is a hard line around 970 cm where the lighting changes and makes the building look absolutely bizarre and not at all like it’s being lit by the sun (The interior of rooms are also inexplicably very dark). I’ve tried upping to production quality lighting, I’ve tried messing with the light mass settings, and I can’t seem to find anyone else who has this issue. I’m assuming I’m missing something very basic. Can anyone illuminate me? (haha)

Was messing around with it, changing the diffuse to zero under the lightmass settings of the box brush I’m using as the level floor seemed to do the trick? What an utterly bizzare lighting interaction…