Remove foliage in auto material at runtime

I am trying to remove some grass of MW auto material, specificly arround a building.
So I made a mask that I update from a BP using MPC :


Then inside AutoMaterial_MASTER I call my mask instead of the original Landscape Layer Sample :

The mask output is directed to multipliers before Landscape Grass node and to the Landscape base color :

The mask is correctly drawn as base color but the foliage removale don’t work :

Please help me figure why it is not working with the Landscape Grass node