One of our licensees encountered this issue. Landscape normal is calculated differently on GPU vs CPU.
See screenshot below: left is GPU (with Edit Layers enabled) and CPU (Layers disabled)
This is a summary of what they discovered about the cause of this problem:
They'd like to know if this is by design, and if so the reason behind it. Either way they want a way to get 'correct' normal on GPU since the CPU normal is of higher quality.
Additionally, if you disable Edit Layers of the left landscape, the result won't immediately change, it still inherits normal from previous GPU result. You need to reimport Heightmap.png or do sculpt editing to 'refresh' the normals.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-95444 in the post.