Planted mushrooms do not spread vertically

According to the Wiki, mushrooms are supposed to spread on a 3x3x3 radius.

In my realm and in creative, mushrooms will only spread on adjacent dirt blocks at the same vertical level (Y coordinate). I tested with a single planted mushroom on a single dirt block at vertical coordinate Y and have a couple adjacent dirt blocks at vertical coordinate Y-1, 0 light level. Mushroom do not spread.