I have noticed that in my world when I do /locate mansion it finds one 8000 blocks away but not the one right next to me. I have added lighting to the mansion, added sea lanterns to the floor and added a railway station underneath it. This could possible have something to do with it.
Thank you for your report!
We're actually already tracking this issue in MC-138887, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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