This one is driving me crazy. Some areas of map have bad edges to the lighting. Adding extra lights in these areas often makes the lighting awkwardly bright and reducing the light map or light bake doesn't fix. Even though I put the overall light map down to 84%, it still persisted with this weird pixelation. Anyone have more experience with this bug?
This usually happens because of a combination of the lightmap being too high and objects being grouped together. Get the lightmap below 100 and ungroup all your objects and it should fix itself.