no texture, layer blending issue

As following this tutorial Unreal Engine 4 Tutorial for Beginners | Free UE4 Training - YouTube
I use layer blending for the terrain however when I try blending more than 3 textures on a terrain “tile” it results in “no texture” ( the default gray-black check pattern ) and never loads until I paint so much of a single texture that it is the only one visible, then it magically instantly loads. I have had this issue in UE 4.20.3 as well as now in 4.21.2 and I cannot seem to find any fix or even mention of this issue.
nothing changes when I play the game.
I have setting on medium. material setting high. painting restrictions set to none.
[upon further investigation]
it seems that it is only caused by mixing Grass Lush and Rock textures.
Am I doing something wrong?
You just paint, I followed the tutorial exactly and his works mine doesn’t. I don’t get it.
Please help!

it is the same issue as here Terrain Painting Issue - Rendering - Epic Developer Community Forums but how do you solve this?