These weird square shapes in the build can happen at different places. Also were different lightmaps join each other there are more strange artefacts that I can get rid of. CPU bakes are fine and are seamless but the GPU lightmass seems to have issues. I’ve tried increasing GPULM samples and lightmap resolutions but they still appear.
If your lightmap UV’s are split where those artifacts are showing up then that’s likely the cause, it probably can’t smooth the lightmap across a separate UV islands. You’d need to have lightmap UV’s where lightmap seams aren’t in places like that.