foliage / shadow error

Hello
I just create simple ground and foliage.
when I rotating a view port for some reason foliage is culling and shadow’ going bright / dark.
can anybody give a clue how to fix this?

fyi, I add this option on engine.ini

r.ViewDistanceScale=1
foliage.LODDistanceScale=1
foliage.DisableCull=1

Thanks in advance