While I cannot comment on this specifically, I will tell you two recent upgrade refund experiences I had. In both cases the refund/credit went to the original source of the upgrade payment.

Last week I upgraded a friend with miles for her birthday trip. She's an economy flyer 100% of the time, so this was a treat for 5 hours of travel. She shares her PNRs with me so I can help in IRROPS, etc. Well, as luck would have it, her first flight of the two was diverted due to terrible DFW storms. She missed her connection, and, in fact, I did get her rebooked while she was sitting on a tarmac in OKC. Of course only economy was left that day.

I assumed she would get the miles since I had gifted the F upgrade, similar to when I buy someone a ticket with miles, and it's their ticket at that point. But within 2-3 days, I woke up to "Your Miles Have Been Reinstated!" (I turned around and gave her the upgrade on the way home, so it worked out nicely).

A few months back, I was refunded an instant upgrade on a seat. That came to me in a Trip Credit, as stated above. Credits like that go to the person who "owns" the ticket.

Upgrades have their own ticket number if they are purchased later, so I think the computer just issues it as it was purchased. If you buy the upgrade, you get the credit back. (Note I am not reading the T&C here, and only sharing my experience since you wanted data points.) AA is very good about refunds (case in point, it auto refunded my miles without any request to do so! I actually directed my friend to request the miles, sure they were hers now!)