it looks like something is wrong with your heightmap baking process. the lower portions of your terrain are being clipped and the upper portions are being smoothed out somehow. it almost looks like you applied a curves effect in photoshop to the end result. what does your process for baking your heightmap look like?
It’s not necessarily the file itself. There is a height limit to somewhere around 500 studs when generating terrain, so if you want to design a realistic landscape, you’re better off doing it the old fashioned way - by hand.