UE4 Landscape material white edge problems

Hello guys

I have a really big issue with my landscape material painting now. When I just paint two texture , one is the base material with LB Alpha Blend and the other one is LB Height Blend, the performance is fine. However, when I add a new Height Blend material and try to paint it with the previous two, each of these two Height Blend material have a strange white edge which looks like a noise texture. what should I do to fix this issue. Really hope to see someone who can help me to solve this problem.