When using bone meal on a mushroom it grows at a 100 % chance. 

If you put the bone meal in a dispenser and create Redstone to activate it, The bone meal only works half the time.

I've recorded this occurring in a unlisted YouTube video As that's the only way I can get the footage off my console.

https://youtu.be/is_TNf27GSI?si=dDPf0pGuvEYiKbEo