Thx for your answer,
no I didn't perform any changes to the surface after the forest creation. Auto update check box for surfaces is checked, doesn't make a difference
It is possible to work like that although it makes it a bit more difficult to paint because I can't see where I am painting
But I am trying to figure out the reason for the strange behaviour as it doesn't make any sense to me. I assume it is somehow linked to the surfaces geometry. I also chencked the poly's normals, they are all correct...