I don't know whether I used the correct form because it is my first time to report bug here. (and I even don't know whether I used the correct grammar because my English is poor)

And here is the bug:

I found this when I'm recording a video. 

1) create a world in 1.12.2

  I used forge and optifine and also a few other light mods , they don't change the world generation , but I don't know if they will do effect to the bug.

seed: 6941964004403099775

2) move around to let the world generate

3) open the world in 21w44a

  after a long time of waiting , I saw the scene:

  sandstone trees!?

  then I moved along the generate edge for a while , and I saw dirt trees (maybe there are also stone trees etc.)