OK, got a nice response from an Alaska CSR on my original issue: A coupon upgrade on the outbound, then a standard 72-hour MVP Gold upgrade on the return, spawning a requirement on the kiosk boarding pass that a coupon be collected for the return as well (even though none is required):

The problem you experienced as been an issue that we have been working on. The message I received back indicates that the problem is fixed for checkin at Vancouver and Las Vegas because they have had the new software installed. In the very near future, we will have the installation process completed throughout the system. We apologize for your difficulty.

So it's a kiosk software glitch, apparently. Those who use an MVP Gold Complimentary Upgrade Coupon on the outbound should be aware, especially if they're upgraded in the standard MVP Gold 72-hour way on the return. Should be fixed soon.