Did you have some service recovery due for any reason? I have been in highest cabin for all of my flights except one since Feb this year when I had a cancelled flight and some knock on problems - and I have been told I have a Space Available Upgrade sitting on my account for when I next don't book in the highest cabin. When I was on the one flight in ET, my BP said DUT/1 on it but there was no space available so I assume it's just hanging there until it can be used.

That said, I have a GUF2 I also can't use so it's not as exciting as it sounds