When red and brown mushroom blocks are placed next to the same kind of mushroom blocks, they form pores on the connected faces. The same happens with mushroom stem blocks.

However, mushroom stem blocks do not change to the pore texture when placed next to a red or brown mushroom block and vice versa. The check for making pores only seems to work for the same kind of block.

To reproduce: place red, brown and stem mushroom blocks next to each of the kinds of mushroom blocks. Break them and observe the pore textures on the faces.
Expected behaviour: red connects to red and stem, brown connects to brown and stem, stem connects to stem, red and brown. Red does NOT connect to brown, brown does NOT connect to red. Connecting means changing the outer texture to the pore texture.
Actual behaviour: red connects to red only, brown connects to brown only, stem connects to stem only.