I tried hunting more than 20 Mushmoms and not even once did it drop Mushmom Spore. I don't know if this is a bug, but even the Headless Horseman dropped it's head more frequently than the Mushmom...
That sounds unbelievably unlucky as the drop rate is a lot higher than 1/20, however the drop rate is not accurate, and this will be looked into for the next update.