Whatsapp Query

Hi all, recent problem with Whatsapp. The "last seen" was not showing for one contact when I signed in. I'd leave a message. It would leave my phone, be received the other end and then the contact would show as online and I would receive a reply. They would go offline and I would see the "last seen", even if I came out of that contact and then go back to it a minute or so later. When I actually exit Whatsapp and then sign in again, the "last seen" has disappeared again. Any ideas?? The other person says they have touched nothing on their phone. It's an i-phone.