I found a video that explains how this material system should be made. The video is by UNDINI at this link 4: Materials Layers - Next-Gen Landscape Material in Unreal - YouTube . It’s really well made.
In summary, the problem is that the Material Layer Blend isn’t able to get the Landscape sample without declaring it in the actual Material, so a workaround is used that lets the Material Layer Stack carry the Sample information using the Custom UVs channel.
The Material is laid out like so:
Then the Sample informations are carried by the Material Layer like this:
And get used in the Material Layer Blend here (in my setup I added a component mask parameter that lets me choose which channel and so which sample to feed to the alpha)
And that’s it