Missing In-Reply-To, References headers when replying with updated subject line

Question

With outlook 2013, if I reply to an email thread while changing the subject, the reply is missing the In-Reply-To and References headers.

Not an issue if the subject is not changed (then In-Reply-To and References headers are there as expected)

Not an issue if using outlook 2007.

Is that a change in outlook 2013, a setting, a bug?

Hutcheson

Good question, also looking for an answer to this. GMail also trashes these header fields if the subject is changed. RFC 5322 (tools.ietf.org/html/rfc5322) does not seem to require this, if anyone knows the reasoning behind this, would be greatly appreciated. – Spectra