Actually there was another thing causing a similar issue with the normals of the landscape.
Here is the post that solved it:
If you have multiple layers in your landscape and (lets say) you have a rock layer that uses a normal map and then you have another layer (grass) that doesnt use a normal map, and then you try to paint both in the same landscape tile, you will get wierd results like seams, wierd tiles that reflect alot or nothing all of the sudden, darker tiles than others, etc.
So if you have a layer with a normal map, plug in normal textures or a vector 3 ( 0,0,1) in all of the other layers.
If you have a layer with a specular map, plug also a “0” or a “1” or a specular map in all the other layers!
Same goes with all the other inputs!