Surface blocks do not generate on terrain which is above y=256. Pretty difficult to reproduce in otherwise-vanilla, but I managed to get it to show up with FarLands 1.6.7 on 1.15.2, where the cutoff can be clearly seen:

Note that this mod combo tends to crash pretty hard when you reach this point, but there is definitely terrain left behind that can be explored after removing the farlands mod.

I honestly have no idea why world gen refuses to work above 256.