I had this issue on a multiplayer server: I travelled out to -6000 via a long nether route following a map I got from a villager and no mansion there. I tried the locate command and it pointed me to the same spot. The server is a few months old and has been updated a couple of times but the region was generated in 1.12 (the mansion was a loooong way out and we hadn't been anywhere close).
Deleting the relevant region file and Manion.dat seems to have fixed the issue, although I haven't inspected the Mansion closely enough to check that it's not chopped in half.
Andrew Downes
I had this issue on a multiplayer server: I travelled out to -6000 via a long nether route following a map I got from a villager and no mansion there. I tried the locate command and it pointed me to the same spot. The server is a few months old and has been updated a couple of times but the region was generated in 1.12 (the mansion was a loooong way out and we hadn't been anywhere close).
Deleting the relevant region file and Manion.dat seems to have fixed the issue, although I haven't inspected the Mansion closely enough to check that it's not chopped in half.