Either it’s because they were placed when in-game material terrain painting was poorly made (2016-2017) or because of texture materials smoothing itself which make such effect but I doubt
I think Gamez2much might have manually painted that dithering!
Back then there was a map-wide “material baking” feature (now the auto brush). The “high quality” option would blur adjacent materials together to help smooth out the automatically painted sections. Unfortunately, it didn’t preserve any data about where materials were manually painted, so a brush strength less than 100% would gradually be lost to blurring. I think he discovered this approach as a clever way to mix the manually painted grass without loss to blurring, not to mention it helps the 3D grass transition nicely, too.