Visualize sculpting your landscape as editing your height map; the lighter the color, the higher the landscape, the darker the color, the lower it is.
Except you can’t go lighter than white, and you can’t go darker than black.
If you export your landscape as a heightmap, I’d wager that the flat area is fully black right now; it’s height is now “0”. You can’t go any lower. If you have to add details there, you must go higher instead.
If you’d lower the contrast of the heightmap in an image editing software, you could gain the necessary values in the dark areas to go a bit deeper. There’s probably a way to do this directly in unreal, but that’s what I’d personally try if I had this issue.
It would make your peaks “lower” relatively speaking, but you can just scale the whole landscape in Z to compensate afterwards.