Unexplained "alopecia" spots with Scatter!

[Giovanni Stengel]

Hi everyone,
I have tried them all!
The only way to fill this hole in the scattering is by removing a large amount of polygons from the surface.
It is not related to the slope option, I checked.
There are no exclude/include.
And Why right there though? It is always there. Even if I clone the surface and I make some editing on it.
I don't understand.
I attached two images, in the second one the render is on orto mode from top to show the entire "alopecia" spot.
Thank you!

[romullus]

Did you check how it looks from underneath? Maybe for some weird reason some of terrain vertex normals got flipped?

[Giovanni Stengel]

No flipped vertexes! I think it is related to polygons density. I don't know.

[romullus]

Would it be possible for you to upload that terrain mesh? No need for assets, just faulty terrain, so someone could take a look at it and possibly figure out what's going on.

[Giovanni Stengel]

For now I solved with optimize modifier on the surface. I will also upload the surface with the problem as soon as I can. Thanks!