Landscape non-destructive layers paint problem

Hi there
Using the new non-destructive layers for landscape editing, sculpt works very fine but paint does not :
all the layers are kind of “mixed” together.
Say you have 2 materials (grass and rock) in a layerblended material, then 2 non-destructive layers, you fill the first one of grass, and paint the second one with rock, the rocks and grass will be “mixed”, I mean the painted rocks won’t be more than about 50% opacity I guess…
Only decresaing the alpha of the “grass” layer to say 0.01 will show me “correctly” the two materials, but that wont be a good way to work with multiples editlayers.
Is there a way to have the top layer totally replacing underlying layers where it is painted ?
Tried this on 4.23 and 4.24, same problem.
The Epic youtube presentation does not seem to have this problem I just don’t understand.
Many thanks