I have the same problem here.
I started UE4 the day it got free, and my BSP materials have since always been really instable, destoying the scale of the material on every BSP in the map as soon as I create, move, or even select a thing in the editor.
Sometimes, it also get BSP faces really dark, but rebuilding the light seems to correct (from time to time) the problem.
Thanks for the dedication.