As you know, the world generates the bedrock layers like the nether, takes its code for generating the nether ceiling, but it was a little too blunt. Any part of the roof, besides blocks that contain bedrock, actually has netherack instead of your usual block that'd be there. The world generation of the certain biome is also completely unreminiscent of the corresponding biome. This is obviously due to the cave aspect of the world generation, and it is not yet accustomed to the cave style world generation. Due to that roof, it causes the whole world under the roof to generate like a cave, and not like the biome under the cave only activating the cave aspect under a certain y-level. In this case, there is no biome generation according to biome, accept for naturally generated structures on top of the ceiling, but other than that, it's all just... underground caves : P . I mean, I see the point that it's a cave style buffet world, but considering that 1.17 and below generate cave style worlds with generation of that biome under a bedrock ceiling, I would consider this a bug within the game, or at least a not yet reached point in the development of the revamped world generation.