Merged actors are dark when the lighting is built

I have a modular building interior and each segment looks different when the lighting is built. My approach to fix this is to merge the actors together that make a single wall to remove the seems. But when the lighting is built the merged actors are black but it looks fine when a point light is lighting it ( like a flashlight).

Why does the lighting change when actors are merged and how can i fix it?

Did you check the lightmap UVs? They should have non-overlapping UVs for the baked light to work.