That makes sense, so frigid Canadian air coming in the OAK and possibly freeze the flapper! My OAK is short on the XXV compared to the P61A, so it's entirely possible. I think I will fabricate a hood for the entry point outside, preventing the wind from entering the pipe while the stove is idle.
Grabbing straws here but if you think this is the issue my suggestion is to add a couple of feet to the flex line so it warms a little prior to entering the stove.
Eric