Not entirely sure what to say, but it seems like the nether sky transforms from red (as expected) to blueish (as expected from the OW) whenever the player goes beyond (±350,±350):

Without knowing any further detail this approximately lines up with the so-called spawn chunks of whateveritis 1.12.+ spawn stuff…

With that being said, the chunk randomness happens at genuine spawn chunks of 1.17.x. https://minecraft.fandom.com/wiki/Spawn_chunk#Behavior. In fact, I would guess that the vanilla 1.14([?, or whatever changed spawning]) changed to spawning conditions. idk it seems(?) to line up with the spawn ticket level or 22 or similar to that. ¯\(ツ)/¯