If a user is responding from a different address than the one to which the email was sent, this is expected.
If we allowed replies to these emails from another email address, we open accounts up to being abused via email with others pretending to be another user. We run into this issue sometimes in our own support messages here on meta.
If contact email forms are in use, they usually send from an email and set the Reply-To header, which means we’re hitting the same problem as stated above.
I’m not sure of a great solution here personally – maybe someone else on the team has a thought.