Hm. Is this with the same map in both 4.10 and 4.11 and the same HammUEr version/lightmap settings?
Do you get the same kind of problems just using generic meshes (cubes stacked on a plane or whatnot)?
I haven’t really changed anything in the lightmap code since… January I think… and all I really do is suggest a size to UE and let it handle all the rest.