I think it’s because of the resolution of your splat map 
e.g https://forums.unrealengine/showthread.php?21269-World-Machine-and-blocky-splats-UE4
“OK, so the secret sauce here is the splatmap size. For whatever reason, you should make the splat / color map around 2 to 4 times the size of the heightmap (factoring here the power of 2 e.g. 2048X2048 on a 1009 terrain) the landscape co-ord will take care of the rest (so set that to the terrain size).”