It's been there since day one.
Not a bug, just a decision regarding how graphics/damage interact. Your missing flap was still generating a ton of excess lift (rolling you to the other wing). The solution? Lower your intact flap to the same level. You'll fly slower, but it'll be easier on the old joystick.
You'll notice on the newest graphics-modeled-planes (i.e. B-25) when a flap is "shot off" the tatters are still there, rather than "now you see it, now it's gone!"