Frankly your setup is a little bizarre- you are running your splat map as a mask through the heightmap layer in your Layer Blend and then using what I assume is the same scale as your landscape. That won’t allow you to paint anything new. Normally you’d tile the height layer to the same scaling as your diffuse to achieve this effect on a smaller scale:
I would suggest trying this approach- it will let you import the splatmap as a weight map and now you can paint over it/replace it, etc.