I acknowledge it would seem to be a Drafting Issue. What does not come clear with me is that the use of a Full Chimney Cap is incorrect. The Contractor who installed it is a long time Liscensed Chimney Sweep,who is listed on the Cap Manufacturers website. Gelco. They also Clearly show the use of Complete Caps covering Both Flues, supplied by Both Furnace and Stoves.
Also there are Countless Homes throughout my neighborhood and beyound , with similar Caps.