I’ve explored a mansion 14000 blocks away from spawn a while ago and a chunk of it at the corner is missing. I’ve seen this happen in other people’s worlds as well and it does not affect every mansion. For instance, the second mansion I explored, 21000 blocks away from spawn, does not have the same issue. However there’s little chance I’ll be able to travel to either of these mansions to take screenshots in the near future due to their remote location from my spawn base. In addition, I’ve made a duplicate of my world and the problem did not exist again. That means the missing corner of the mansion is not exclusive to the seed of the word I’m playing on.
Duplicate of MC-96998 (I may have falsely said that the bug was fixed in 1.13, but I have also been able to reproduce it by using the seed -1712579024308203924 and teleporting to -5440 ~ -11184, then walking east until I step on plains, roofed forest, or birch forest. When I stepped on those biomes, I exited the game on every chunk boundary until I saw a mansion that is cut off. This was reproduced in 1.13.2 but I cannot reproduce it in 19w09a)