When loading a 1.12.2 world in snapshots 18w19 or newer some random chunks mutating to new chunks with the new 1.13-bioms and deleting all blocks and bioms that were there before. The minecraft game output says "Chunk file at xx,xx is missing level data, skipping" about 2000 times. but when the affected chunks are recently loaded in the version 1.12.2 by flying over it, the problem is not as bad as before.