Save taken just before a goblin siege arrives (same calendar day). The siege then departs on the next tick. This is not the first siege at this fort - all others have progressed normally, were defeated primarily with cage traps, and the captured siegers released. Difficulty settings were used to disable megabeast attacks, werebeasts, and curious wildlife, and various other settings were also changed. No mods were used.

Edit: it appears (based on their thoughts) that the siegers include one or more goblins who were previously captured and released, or who previously fled when others were captured, suggesting that they retain their fleeing state when they return. I hypothesise that these are squad leaders, thus the followers also flee. This is consistent with my observations of later sieges on this fort, where some goblins flee but others attack as normal.