BlendMaterialAttribute node not working if used in landscape layer blend

Hi, I’m trying to make the landscape to change in growing areas around my shrines at runtime.
Since I’m not experienced in landscape and material, I thought I would first try something simpler to understand how it works : have one of my landscape layer to be a blend of 2 material that I could change at runtime.
I make my 2 materials, I use make material attribute on each.
When I link the first one to my layer, everything is grass, which is expected.
When I link the second one, everything is lava as expected.
When I use BlendMaterialAtribute node, with an alpha of 0.5, the preview is a blend of my 2 previous material, but when I link that to the layer, the result in my laver is a checker board texture.


Do you know what could cause this issue?