Lightmass flicker, inconsistent. Construction script, Instances used.

What is going on with my static lighting?
Here are 2 screenshots, taken in succession. Lighting has been built, but shows 2 results and alternates them.

More about my setup and what I have attempted:

Each room is a Blueprint, which Adds Instance meshes in its Construction Script. The floor and roof are Child Actors; I scale the floor/roof UV tiling to match the room size, and am under the impression that I can not achieve that with Instances. The walls are all Instances. The 4 Volumes seen are Door Blueprints added as Child Actors in the Construction. All assets have been set to Static, except the Doors which have “cast static shadow” turned off.

I’ve tried changing the Bounds Scale, changing mobility settings, ensured all the materials have “use with static lighting” and “use with instance static lighting” where applicable. 1-sided meshes have “cast 2 sided shadow” turned on. Please, if there is something I’m missing, I’d love the assistance.

Why so dark? Note there are 4 point lights in this room.

Suddenly, the lighting is different. I nudged my viewport camera and things are different! Though still the strange squareness in the lighting?