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.
Linked issues
is duplicated by 4
MC-127119
Structures misbehaving from previous versions to new versions
Resolved
MC-127376
Guardians not spawning.in old world and new world generations.
Resolved
MC-127396
Guardian's do not spawn in farms.
Resolved
MC-129626
when switching world from 18w16 to 18w20 guardian farm doessent spawn guardians anymore
Resolved
relates to 2
Attachments
Comments 14
I added screenshots what happens if you create a world with said seed and tp to said location for each version separately (18w07c and 1.12.2). 18w07c didn't create Monument.dat file yet the monument generated when teleported to this place. Did the location of saved structures changed?
Oh. This should be updated with the more recent versions of 1.13 snapshots. My search, https://bugs.mojang.com/browse/MC-126943?jql=affectedVersion%20in%20(%22Minecraft%2018w10c%22%2C%20%22Minecraft%2018w10d%22)%20AND%20text%20~%20%22structure%22 yielded no results and the versions listed here are not selectable currently in the search options. This happens in 18w10c & 18w10d currently.
Added. It is possible to search for older versions, though, but only on the advanced search; see WEB-929.
It's generally better to not assume tickets are up to date, limiting the ticket creation range to the last X weeks usually works for new issues.
Here is a twitter link where I explain how to convert old witch hut structures to the 1.13 snapshots.
The structures do not generate completely because when converting from old worlds to the snapshots the value for the reference to the start chunk of the structure is not created. In case of a larger structure such as an ocean monument it applies to all chunks where the structure is included.
[media]Sometimes the "structure tag" is corrupt or do not generate in the start chunk of the structure.
[media]
gnembon
migrated
Same issue with 18w07c. Loaded 1.12.2 word into snapshot to see new mechanics and how they would affect farms. No guardians spawn in existing guardian farm/temple. Tons of hostile mobs spawning though, so I am assuming the game no longer recognizes this as an ocean monument.