Confusion About Mixed Terrain Sets

Hello!

I am very new to working with tiled and I am kind of confused about how terrain sets work. To experiment and learn I have drawn the following tileset:


Since the grassy edges connect at the corners while the dirty centers connect at the midpoints of edges, I thought I should use a mixed terrain set as follows:

However this does not seem to work correctly with the terrain stamps.

I don’t quite understand what I’m missing. Any help is appreciated.

Thanks!