The layer blending needs to be changed from Weight Blend to Alpha Blend
In your Landscape Material, set the Blend Type to LB Alpha Blend for all the affected layers
In the Landscape Edit Mode, create Layer Info of the Non Weight-Blended Layer type for all the affected layers (if you have already created the later info, open the “XXX_LayerInfo” Landscape Layer Asset and enable the No Weight Blend checkbox)
In the following video (at the 1 hour 14 min mark) you can see Paulo is using the setup I just tried to describe:
My understanding is using the Blueprint Brushes to paint Landscape Material Players is analogous to manually painting Landscape Material Layers on separate Edit Layers. If the Material Layers are set to Weight Blend and painted on separate Edit Layers, they will each get a 50% share of the blend weight and you will see both of them blended together. If Weight Blended Landscape Material Layers are painted in the same Edit Layer, one can have up to 100% of the weight causing only it to be visible and the other to disappear.
The solution to both of these issues is using Alpha Blend in conjunction with the Non Weight-Blended Layer type.
If you need a visual explanation of what I just said, watch this video: