Here is more from the Wall Street Journal:
Responsible Statecraft, a publication of left-leaning Quincy Institute, reports, that in November a staffer at the Institute for the Study of War was fired for editing a map to show that Russian forces had taken the Ukrainian town of Myrnohrad. The employee knew that Polymarket often used the institute's maps to settle bets about military outcomes. By fabricating a Russian advance, according to the report, the staffer triggered payouts as high as 33,000% on bets that Russia would take the town by Nov. 15."
Here is Responsible Statecraft's piece on the incident.