ohhhh I had no idea you were using custom UVs. It is because MatLayerBlend_Standard does not even blend customUVs, only the most basic channels.
Try hooking up your landscape coords to custom UVs on the final material input node itself. That means do not use MaterailAttributes as the final pin. My guess is that its being lost somehow by the attributes nodes.
Also, with your above setup, it will give crazy results if your landsacpe coords are scaled differently in any functions. In order to blend between two textures with different tiling, you still need the coordinates for the entire area for each layer, not a final blend of all the coordinates like that setup would create. Otherwise the textures will drastically change scale and warp as they transition to eachother.