World: Customised World with following settings: River Size 10, Sea Level 64, Biome Desert, Height Stretch 100. Seed: -7170449905555944033. Coordinates: -589X, -3023Z.

Issue: Described in screenshot. Grassy chunks are apparently ocean biome.
This is NOT the bug where chunks from a deleted world of the same name appear. I know this for two reasons. A) I have never had a world of this name before. B) These anomalous chunks do not create cliffs as they usually do, but instead follow the sand terrain as though created by a WorldEdit //replace command.