I have noticed recently an issue with CC emails. If a person you CC'd responds, but not 'reply all' (meaning the CC just responded to you), you will get the response... but if you reply, it sends to the primary contact instead of the person that actually replied!

Support in ticket 2643675 said this is as expected, since the communication is technically still tied to the primary contact (which is true) but there should DEFINITELY be some sort of alert letting you know you are responding to the primary contact and not the CC!