Mongol gets resources from attacking unfinished buildings, either from destroying it or from the other player cancelling it (even if only a single shot from the khan). They are supposed to get resources from setting building on fire according to the game, not from landing one shot on unfinished buildings.
Any attempt to deny the plunder from Mongol by deleting a finished building that’s under attack does not work, and the Mongol player will still get their plunder, which is the correct behavior. But I suspect this bug is from a poor implementation of that logic.
With this bug, Mongol will always have the advantage in tower rush, even if you increase their outpost cost to 200 wood.
Game version: 5.0.10257.0.