(it might also exist in other 1.7-1.12 versions

1) create a world in 1.8.9

seed:6941964004403099775 (没错还是那个种子x)

2) set render distance to 12 Normal+

3) go to mushroom island

/tp 3065 100 -8165

4) before the world finishs loading up , set render distance to 2 Tiny

5) exit the world and open it in 21w44a

6) set render distance to 12

and I found that there were only brown mushrooms and no red mushrooms

it is very strange

after that , I tried a few times and succeeded 3 times

It seems that the old chunks only generate one kind of mushrooms and the new chunks generate "dirt mushroom"