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).
- 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.