The bug

If a world using custom generation has Mansion generation enabled but has noise settings such that no terrain generates anywhere, then leveling up a cartographer to Journeyman or typing the command /locate mansion causes a tick freeze that prevents debug crashes from working. The same applies with /locate nether_fossil in the Nether, and /locate endcity in the End.

How to reproduce

1. Open the attached world, or generate a world using the Import Settings button and the attached generation preset. (The attached world has a normal Nether and End.) The game mode must be Creative or Spectator to prevent an infinite death loop.
2. Execute the command /locate mansion, or level up a cartographer villager to Journeyman
   → Entities and some other processes stop working, and trying to force a debug crash never finishes, similarly to the effects of MC-175269.

Note 1: In the attached world, teleporting to the coordinates where the same seed produces a known Woodland Mansion reveals there is no mansion in sight.

Note 2: I have tested all other structures to see which ones cause the issue.

• Nether Fossils and End Cities do not generate and cause the same issue as Mansions when using /locate.
• Buried Treasure does not generate a chest. Depending on the exact world generator settings used, /locate buried_treasure will either output "Could not find the structure nearby", or will point to where a buried treasure chest would be generated normally. In the former case, any treasure maps generated from Shipwreck or Ocean Ruin loot will be replaced by empty maps.
• Mineshafts and Strongholds do not generate, and /locate mineshaft outputs "Could not find the structure nearby". Eyes of Ender do nothing.
• Ocean monuments still generate, surrounded by a cuboid of water. Ocean explorer maps are not broken.
• Ruined Portals generate in the Nether but not in the Overworld; /locate ruined_portal finds a location in both dimensions, but nothing is generated at said location in the Overworld.
• Bastion Remnants float, with the bottom near y:31.
• All other structures generate on top of the void and are findable via /locate as normal. Underground parts of certain structures, such as igloo basements, do not generate. In 20w46a, the bottom two blocks of buildings are missing; the lowest generated block is at y:1.