In a newly-created map, near spawn there is a very large Mesa biome that contains a smaller Bryce variant with a river running through it. Near the edges of the river, the edges of the Bryce variant appear to stop generating blocks normally over the water. That is, some of the land sits atop the water without actually continuing to where you'd expect its base to be.

Steps:
Create new world using seed 416085818641294396
Teleport to 248 63 -38
Swim around in the water looking up at the edge of the Bryce area.

This was in a recreated map, so I noted the seed and coordinates, deleted the previous world, generated a new world using this seed, teleported to the coordinates and confirmed that this happens consistently (for me).