That's how RFC2822 defines the email headers. That's not a leak, that's just how email works. When you send from the web app it uses that client as the originator.
I'm unsure how many journalists know that their replies using an e-mail client will send their ip address, neither if they can understand why there is a difference between the mail client and their web interface.
I only know one organisation using fastmail services today, and I asked if they knew about this today, which of course they didn't. There surely are reasons to not break the RFC as gmail and others did, but the expectations from users need to be addressed somehow