I had posted this in a separate thread previously, but now that I know it's indeed a bug...

I was attempting to book three award seats for family members. Once the departure and return were selected, I had to log in to continue. At that point, this message would appear above the passenger fields:

"Because this itinerary was created using Medallion® availability, the names of the Medallion passengers can't be changed.
Medallion must be traveling to receive greater award seat availability."

This would happen whether I had logged in at the beginning of the search for seats or not. And I'm not even a Medallion! So I called Delta and was put through to web CS, and the rep got the same message when she tried to book a seat using her own account. Which too, was not Medallion. She put the seats through for me, but had no idea why this happening or when it would be resolved.