The bug
In the new snapshot, structures seem to generate at entirely different places than previously. For example, seed 1 has a jungle temple at x=744, z=56 in 1.12 which is not generated in 18w06a, therefore 18w06a has one at x=520, z=312 that did not exist in 1.12.2
Structures that are affected:
Jungle temple
Igloo
Mineshaft
Stronghold
Witch huts
Structures that are not affected:
Desert pyramid
Villages (Layout might be slightly different, tho)
Ocean monuments
Linked issues
is duplicated by 5
MC-127176
dungeons with spawners not at same location
Resolved
MC-128983
Witch huts and other structures don't generate properly
Resolved
MC-129952
Jungle Temples and Witch Huts Missing
Resolved
MC-132246
Witch hut failed to generate
Resolved
MC-134720
Some endcities don't generate in 1.13 while they do in 1.12.2
Resolved
NeunEinser
migrated
The worldgen refactor changed the way decorations are seeded, so changes are expected.
Structures that were partially generated pre-18w06a should still generate at the proper location.