"Working to track down the culprit"
Well, here's the idiot who vandalised the OpenStreetMap data:
https://www.openstreetmap.org/user/MedwedianPresident/history
...and is currently banned until 2038.
The edits were made 20 days ago, and were reverted within a few hours. But presumably Mapbox grabbed the changes in the brief window they were live (or accepted the edits but not the reverts).
I actually wonder if Mapbox's filtering of changes actually worked against them on this one, as the OpenStreetMap community undid the vandalism pretty quickly.
Biting the hand that feeds IT
