While composing a reply to recipient B leaking some details that it "learned" when reading a mail from sender A, which you did not want to share with B. I have no idea how they organize sessions, indexes and whatever they use. But if no "side-channels" existed, I would be extremely surprised.

Of course reading generated text remains the sole responsibility of the user before clicking "Send". We all know that reading drafts can happen more or less carefully, especially when being in a hurry.