World Generation Bug

Bamboo spawning underground, on the wrong block

Steps to Reproduce:

[Step 1] The world was created initially in 1.16 as a paper server.
[Step 2] The world was updated to  1.17 only when the full release came out. Still presently played as a server.
[Step 3] I went exploring for new blocks and generated new chuncks, in one of the caves i explored i found the bamboo naturally generated there. 

Observed Results:

The biome was a bamboo forest (on the surface) yet somehow one growth of bamboo spawned on a gravel block around y~20

Notes:

I broke the bamboo on the second block up and all the blocks above that broke like normal; no update on the bamboo piece that was on the gravel. The bamboo did not update until the gravel underneath was broken.