This is nuts. I planted brown and red mushrooms on surface level mycilia. They will not reproduce and spread. So I grabbed a silk touch pick and gathered some mycilia blocks. I tunneled out underground in the mushroom biome. I then planted the mycilia
blocks. I then planted both brown and red mushrooms in this underground room. No light. Door closed off tunnel. They STILL won't grow and spread.
I put some plain old dirt down on the floor of a cavern in a normal biome. Planted red and brown mushrooms, and they grow. What the heck? I put mycilia blocks down next to regular dirt blocks. It spreads. I built a room on top to shut out light. Planted mushrooms. They don't grow.
The Wiki says "Mushrooms spread faster on Mycelium than on other blocks". Yet in my world, they don't spread at all on mycillia blocks.
What the heck, mushrooms grow normally in non-mushroom biome but refuse to grow in the mushroom biome. That makes NO sense. None. There were mushrooms on the surface in the biome when the world was generated. Not one has spread.