How do I paint Terrain in UEFN? (Is this a bug?)

I go to the pain section and paint with the desired material.

The issue is that certain materials cannot be overlapped.

Picture demonstrates me trying to pain grass over gravel. I don’t know if this is a bug, or the intended way of replacing paint is something different from what I am doing.

Each layer should have a weight. Grass has lower weight than gravel so cannot be painted over gravel.

To remove gravel hold shift while the gravel layer is selected and left click to paint.