The bug

Information about previous structures (generated in the world) saved in data/*.dat files seem not to load when a 1.12.2 and below world is opened with 1.13 (tested on client and server).

How to reproduce

Run 1.12.2 client, create a new world, seed 2791111690993685248 Teleport to about 5256.0 65.0 -248.0 Observe guardians frolicking in the autumn mist Close 1.12.2 Open 1.13 and enter the world Fly up to Y190 to let the existing hostiles despawn Descend to Y60 Observer no more guardians to spawn Confirm the observation typing /kill @e[type=minecraft:guardian]

The ability to store previously generated structures, even if the newer world generation/format had different opinion about it, allowed for players with stubborn personalities to continue playing in their old and dingy worlds even in the newest and shiniest of Minecraft versions.