How to loot the Routiers' camp for the event challenge?

I’ve tried this mission twice now, but the game still says I haven’t done it. I destroyed the Routiers’ camp, and the second time, I found some gold I could collect, but it still isn’t showing as completed. Has anyone worked out how to complete it?

Okay, I’ve got this to work now. The first time I’d done it, I had destroyed the camp but not picked up the gold. When I did it again I continued from the save and picked up the gold, but that didn’t trigger it. I just did it again, restarting that campaign mission, and destroying the camp and picking up the gold, and that has triggered it.