Lightmap's building bug?

Hello all
Thank you for this great software! Thank you for opening code!

I compiled UE-4.10.1 on my gentoo linux and learning UE
I have a problen with baking light maps
http://wstaw.org/m/2015/12/17/plasma-desktopyJ4841.png
Look at this sharp shadow shapes.
When i check/uncheck “Overridden Light Map Res” or change it value all become fine
http://wstaw.org/m/2015/12/17/plasma-desktopGM4841.png

Is this bug?