Landscape material problem - Strange line because of lightning

Hello, I have a problem with a strange line that appears when I create my landscape material, and I cannot make it go away no matter what I have tried (changing preview weight etc.) It looks like this:

I have highlighted the strange area with a blue brush.

Do do you have a picture of it on your landscape?

In between the two blue lines, the exact same line can be seen. Although, I will say that it is a lot harder to make appear when it is a completely frozen picture, instead of when you move around in the game. You might have get a bit closer to the screen, but it should be visible.