Adding to what others already mentioned. The Landscape actor, that you place in your scene, has a transform. (translate, rotate, scale) The default scale values are set to 100. The Z-axis, the blue value, is the height scale. By default, 100 scale gives you -256 to 256 meters in engine. Let’s say you need a mountain height range of 1 km, you would need to set the Z-scale to 200. Which gives you -512 to 512 meters.
Your sculpted values never change however. Like @chrudimer said, you are painting a 16 bit grayscale map. These values are translated to whatever scale you’re using. Say that you painted the max value and it flattens out on top. Increasing the scale will not remove this flattening, because the values are the same. You then instead have a greater range, and would have to adjust everything you have sculpted.
Therefore it is important that you understand what Z-scale you are using in Unreal, and what size it represents. Because not only do you need to set this from the start, but you also need the height range when creating heightmaps in another software.