I'm not sure I understand you correctly.
When you click on a header field it gets filled with some data from another (header) field?
Or do you mean to say that when you press the icon with two 'V's you're getting the value of the same header field, but from a previous message. The latter is what's supposed to happen as that is the history button.
When you want to insert data from the address book into the To:, CC: or BCC: fields, then you'll need to press the icon with the 'contact card at the útmost right of the mentioned fields.
When none of this makes any sense then I'm probably missing the point of your question, but then I don't understand why you'd want to insert data from your address book into your Subject: header.
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I'm just a user of The Bat! I don't work for Ritlabs.