Broken lighting on unlit static mesh

Do you have a lightmass importance volume covering the level?

Hello,

I have been making static meshes from BSP brushes and am having a serious lighting issue. As you can see in the following screenshot, if my mesh is not directly lit from the sun (i.e. in the shadow of a building), the lighting of the mesh “breaks”.

Now if you look at the meshes on the other side of the street where they receive the sunlight, the lighting is fine.

What is causing this issue and how can I fix it?
Thanks in advance.

Yes, I have one, it is surrounding the whole level.