I have a fully explored world from legacy console edition (about 2544x2544) in which I haven't been outside that area awaiting 1.18 and having completely new chunks generate outside that area.
There seems to be an issue where subchunks are only created up to a circular area (red circle in the map below) of 2440 in old chunks, leaving an area of void visible from y 0 to -64 in the rest of my old chunks where they failed to generate (outside the red circle in the map I attached)
New chunks (y= 320 to -64) are generated correctly outside of my 2544x2544 legacy console edition area (white area around the map)
Teleporting to around 2544 in any direction and going below bedrock will allow you to see the issue.
Also teleporting to 0, y, 0 and walking below bedrock shows that subchunks generate below y=0 correctly until you reach to about the red circle represented in the map below.
Currently uploading a copy of my world so you can check it yourselves, as well as a clip walking from 0 to 2500ish to show the issue. Will add links as soon as they´re up.
World download (from 1.17 releases, upgradable to 1.18 beta): https://1drv.ms/u/s!ArG7tvnDDwS4mn7haTnCQktqA4Mz?e=rrPCGy
Going from 0, y, 0 all the way to 2544 (new chunks) and showing how subchunks fail to generate after a certain distance (2450~): https://youtu.be/H2S2uGQDv_g (watch at 2x speed)