I also checked this using the Ask Me Anything assistant in the Orchestrators Portal, and I came to the same conclusion as Cameron.
Based on the available documentation, callbacks are treated as outbound interactions, and AHT includes Talk Time, Hold Time, ACW, Dialing Time, and Contacting Time.
However, I couldn't find any documentation that explicitly confirms whether the preview countdown timer or the post callback countdown timer are included in the AHT calculation.
So I completely agree with Cameron's recommendation. If you need a definitive answer for this specific behavior, opening a Customer Care case is probably the best approach. That way, Engineering can confirm the behavior, and hopefully the documentation can be updated to clarify it for everyone.
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Raphael Poliesi
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