about custom mesh as landscape collision

Hello, sorry to bother you again.
I encounter a strange behaviour with collision while trying to use a static mesh as landscape.
Sometimes it collides, sometimes not, especially in the area I higlighted in the picture attached. The mesh is 38600 polygons (77k triangles)
The collision is set complex as simple, I tried almost every setting possible, but with no results.
any suggestion?
I would like to avoid using the custom landscape tool because it would be a pain in the ** doing what I’m trying to do.

**edit: **as far as I understood it’s not possible to have a static mesh as landscape. am I right?

**edit2 : **apparently I have the same problem even if I create a default landscape. so the ball I’m using as actor if it is travelling at low speed it will collide with the mesh, but if it goes faster it will break the collision. I dont know what to do.