You're confusing YUP fares (which usually book into A if it's available) with complimentary upgrades from full Y fares for Medallions. For the latter to happen, there must be appropriate upgrade inventory (V for most flights with the premium cabin marketed as first class, X for flights with the premium cabin marketed as business) for you to upgrade into at the time of purchase.

Your outbound has no V inventory (or X as above) so even though you booked full Y, you're waitlisted for that V (or X) inventory to open up.