I thought I would update my gold farm in the nether by adding a small bamboo farm so the furnace smelting all those gold swords could self-fuel.

However, I found a snag I was not expecting. Zombie Pigmen started to spawn inside the farm, on the dirt being used to grow the bamboo. (Picture 1, ZPs on dirt with freshly planted bamboo).

I am not aware of any other vegetation that mobs are allowed to spawn in, except things with no hitbox. Is this behavior as intended, or is this a bug?

UPDATE #1:

I bonemealed the bamboo in the farm so it was all at least 2 blocks high, went > 128 blocks away and returned slowly. New Zombie Pigmen spawned in between the bamboo. (Picture 2, ZPs spawned after bonemealing the bamboo).

(BTW I also noticed Zombie Pigmen spawning on the scaffolding. Very irritating, but no consequences for the farm. Picture 3, ZPs spawned on scaffolding).

UPDATE #2:

After the bamboo had grown some more no more Zombie Pigmen have been seen spawning in the farm. Mobs spawning on bamboo only seems to happen during the early stages of growth. But should it be happening at all?