Lightbuild randomly lost, Looks good but FPS drop

So we’ve been having this problem with this project now for a while. it consists of 26 apartments.

The Problem:
We randomly get “LIGHTING NEEDS TO BE REBUILT XXXX unbuilt objects.” this can happen by anything from reloading a level, changing a collider or anything like that, that shouldnt affect lightbuild.
We can do a 3 hour bake, everything looks well and when we save and reopen the level, everything still looks good, but we get the Lighting message and huge FPS drops. this occurs through all of our levels in the project.

What we’ve tried:

When we use DumpUnbuiltLightinterations, what seems to pop up is a blueprint with a couple of static meshes and 3 static lights.

*The BP is all on Always Spawn ignore collision and nothing is tampered with in Eventgraph nor the constructionscript.
*We’ve tried rebuilding the BP from scratch, but no changes, we still loose the build at random.
*We tried removing saved everything but Content and config folders.
*We tried copy level to another project, we think it might work there, but since the problem happens sporadicly over the 26 apartments, we cant tell for sure.
*We tried duplicating the level, but still not working

you cant see the red text, because my screensnippet tool unfocuses UE4

Also, the preview overlay text only appear when i get close to the lights in question!

Far Away

Closeup

the fps drop is because the engine is switching to dynamic lighting when the lightmaps are lost. that also causes the “preview” splat to show up. i don’t know why reloading a level or changing collision would destroy lightmap information though but i have seen other posts where people have similar issues.