It happens with all heightmaps, just depends on how u make it. If its not smooth color then it will bump like urs is doing. Meaning, to make a hill on flat land the land is all one color and a smooth transition should go into the hill, a transition u will barely see change from one color to another. Heightmaps are affected by width and height as well, the higher u make ur map the more bumpy it will be, but if u add width to that map it will smooth out the terrain. Any speckles of grayscale on the heightmap png bsically reads as bumps/hills/mountains/rocky jagged edges
Quick note: If u dont want a super large map, try working on making dark/lighter increases on ur heightmap. If you set the land to a dark gray and the hills a shade of grey maybe 3-5x brighter and smooth transition then u dont need to worry bout the height as much
