Bamboo generates in caves under jungles

Bamboo spawns on gravel in caves and caverns under jungle biomes. This does not happen in bamboo jungle biomes at the same rate as the bamboo spawns above ground.

It is also most likely pretty rare, as bamboo spawning in regular jungles is decently rare, and gravel is also uncommon. I have only found about two of these in the 30 minutes i've been searching in spectator mode.

To reproduce

(from MC-215880)

Seed: 8433902526619557614
Coordinates: 91 27 438