Chunks being reset

Just went to play my world, and had a HUGE UGLY SCAR generate across my world in the form of a large L-shaped mess of chunks resetting to a mesa biome.  These chunks should be a nice ocean biome, alongside some swamp, forest, and plains biomes, but now the chunks are all messed up.  Here, I've included some pictures from MCASelector showing what is happening.  The first picture shows what these chunks should be, and the second shows what is happening to these chunks even after a put them back to normal using MCASelector.  Get this fixed ASAP, by the next snapshot, please.  I am really not happy.  Thank you.