I am currently trying to turn a mushroom island green by planting brown mushrooms in a grid pattern (6 blocks of spacing between each stem of a brown mushroom) to kill the mycelium by lack of light so grass can expand. The mycelium is unfortunately not turning to dirt after update 1.16.0. I did this to a different island right before 1.16.0 was released therefore I believe this is a new bug introduced after the update.

Update: This might be a tick speed bug due to the fact that grass blocks are not being overtaken by mycelium, mycelium and grass blocks sometimes spread to dirt but hardly ever, and in the past 10 minutes my 5 planted sugarcane have not grown once.

2nd Update: I was digging up grass in another biome (I believe plains) and the grass was spreading over the dirt normally. I then went 4000 blocks away to another mushroom island to see if it was the spawn point mushroom island that was bugged but the new island was doing the same thing as stated above. I believe this is a mushroom biome specific bug but I'm no expert.