Some Chunks in nether Sink for 64 blocks

Some Chunks in nether sink.As I stood on the ceiling bedrocks,it was at y=64.The I checked the floor bedrocks,It'd disappeared to voids(below y=0).

Associating with previous chunk bugs in 1.17,I did some purposes:

①.Chunks&blocks in nether limit in y∈[0,128]

②.The height of chunks in nether is 128.

③.Sank Chunks probably want to obey rules like chunks in overworld(generating at/over y=-64)

_So,According to ①,chunks in nether are not allowed to do generates in y∈[-64,0],so the attempt always fail,and void eats instead ;_

_According to ②&③,-64+128=64,so the top is at y=64._

So, the floors don't exist in these chunks and the ceilings are at y=63.

That seems to be rational,doesn't it?

*So what causes ③ and engenders this bug?*