CPU lightmass processes every mesh on a different thread, that’s what creates these discontinuities. It’s expected.
A very long time ago, there was some discussion about implementing lightmass groups for meshes so that they would all bake as one, but this was never implemented. Probably because it makes more sense to offload it to the GPU… but Epic didn’t really do that either.
Daniel Wright mentioned that Epic’s solution internally (which they were not happy with) was to increase the lightmass quality, effectively a brute force approach. This is done by lowering the level lighting scale/lighting smoothness and increasing the indirect lighting quality. Ideally you push these only as far as you need to go, to avoid unnecessarily long build times.
You can read more about it in this ancient manuscript: Baked Lighting Variation Between Static Meshes
Also you pretty much need to be baking on production quality. Anything lower isn’t expected to produce shippable quality.
