This has always been a problem.
People do different things to get around this.
To my experience, the problem comes from both landscape material and (sometimes) the grass material.
The UE4 documents say that you should not use a specular map for non metals. Well until your geometry doesn’t have millions and millions of triangles that statement is wrong and you should use a specular map. A specular map will compensate for all the tiny holes, cracks and things like that on the surface where it’s actually having a little depth and is not reflecting the light directly to your face like a mirror would do. Generally it doesn’t make much of a difference but when it comes to landscapes it shows the error very clearly.
Second issue is that you should then adjust your grass specular to match the specular on landscape. Activate specular view in the buffer to see better. Then also make sure your grass SSS isn’t very high because landscape doesn’t have SSS and so while the light makes the grass look brighter on the opposite directions, your landscape brightness doesn’t change. Usually using the base color as SSS without increasing it’s intensity gives the best result.