I have 2 adjacent poly surfaces, basicly 2 planar floors each belonging to a distinct room.
When I render, everything looks seamless specially where the 2 floor meet but when I bake the lightmap there a difference in luminosity generating an apparent seam. the 2 floors where baked with the same FG solution and with the same lighting setup .

If I turn off FG, there is no seam, freezing the FG map bwtween the 2 bakes didn't solve the problem, only if I combine those 2 floors together (without merging vertices and UVs) the seam goes away when baking them together at once .

Combining all my level when baking is neither doable neither affordable (it's taking me from6 to 12 hours to bake a room). why am I having this difference in luminosity when having same fg/light/materials? why combing the 2 pieces solved the problem even with FG rebuild ON? Whats the difference when baking the 2 floors one after another and when doing it together in 1 shot and 1 mesh ?why is this happening and how can I fix it?