So, what do you think Bieber is up to in his "weeks at a time" treks into Oberslecht? He must know about beastmen.
I think he avoids beastmen, using his skills to stay clear . He keeps quiet because he knows there are enough of them to be a serious problem for the area and he doesn't want hot-head riling them up. For some reason they don't like coming west (e.g., in the chase scene give up chase) and let's not stir them up.
He can give advice that helps cope with some of the Oberslecht's problems (midge-bane ointment perhaps) but only to those who accept his guiding services which will keep them away from beastmen (and thus from the area of hag tree etc.). He doesn't fraternize with the farmers in that area, considers them all loco.