After tracking down a woodland Mansion, disappointing to find it was only a one block thick facade. Perhaps caused by it having spawned on the edge of an area? The symbol for the mansion did appear on the extreme edge of the Woodland Mansion Explorer map.
Related issues
duplicates
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migrated
Was the area anywhere near it generated before 1.11? That's the most likely reason. AFAIK it doesn't have anything to do with the edge of a map.