Back in the early days of Minecraft - Pocket Edition, you were able to generate worlds using a world type called "Old", this generated a 256x256 world, and was intended for low end devices at the time. Over the years new updates for the game came out and eventually that "Old" world type ended up being removed as an option entirely, however a problem occurred starting in v1.17.0 and is still present currently in 1.19.0. Whenever you load up these much older worlds in these newer versions of the game, a portion of the original world appears shifted over so much that a good portion of it gets cut off by the invisible bedrock barrier, new chunks will appear to be present as well outside the older chunks.

Upon further investigation on the matter, I figured out what the potential issue might be, back in the day "Old" worlds had a default spawn point of X: 128, Y: 64, Z: 128, "Old" worlds made in much later versions of the game such as 1.17.0+ do not have a default set spawn point, instead the spawn point varies per seed much like it does for infinite worlds. The issue here is the game isn't using the default spawn coordinates that the older worlds have set in place and instead forces the invisible bedrock barrier to shift according the to spawn point that's provided by the modern algorithm, as a result, the older world will appear shifted out of place. Attempting to change the spawn coordinates back to its original default coordinates doesn't fix the issue as the game seems to be programmed to set the invisible bedrock barriers in place based on a "first time load after update" mechanic of sorts whenever you update the game and load the world, as a result of this the game will force the modern algorithm spawn coordinates back when you load the world.

It's hard to say if the spawn point is really the issue, but based on some tests that seems to be the case, the images speak for themselves though, I highlighted where the old chunks are and the other chunks surrounding the old chunks are the freshly generated chunks.