I had a world called "My Land, My Rules" in Custom mode. He had NO ocean monuments (no Guardian farm) but all of a sudden he was DELETED! I created a new world (which had oceanic monuments), but it was kind of weird, as some chunks had been corrupted and there was a desert temple in the MIDDLE OF THE OCEAN, MORE OR LESS NEAR A BUMPED CHUNK OF SAVANNA! And when I saw ... The old My Land, My Rules has collided with the other world! But there was a chunk near my old base that had been corrupted. By the way, the two worlds had been created in 1.12.2, and continue in this version.