After a lot of reading, what I understand is that the problem comes from the fact that specular reflections are based on normals.
If you use a strong normal map with a lot of details in a material with a roughness of 1, it will be less shiny than a material with a roughness of 1 too but with a flat normalmap or having few details in it.
The problem is more noticable in the distance because, due to mipmaps, the normalmap is losing even more details.
Composite texture is not really helping here. It’s usefull for rougness map losing details in the distance because of its own mipmaps but it has nothing to do with the difference of shininess of two 1.0 rough materials, it just helps to keep detailed roughness map in the distance but doesn’t break specular like the normalmap do.
It’s not specific to landscape. It’s more noticable on landscape because it’s more flat than most meshes (I get almost no specular on grass meshes with a lot of blades unless I turn off “tangent space normal” in the material settings which breaks normals)