I booked the ATL-LIT flight the day before. At booking, only middle Delta Comfort seats so I tried to specify only wanted upgrade if Aisle or Window but nothing was working. Next thing you know, I had been upgraded to middle seat - so that was happening at same time I was trying to change it. I went back in and moved myself back to 21C, and with that dropped off the upgrade list. When the upgrade happened next day en route, I was surprised. The flight was in the app when I went up to the Agent because I had the seat map in hand. After they moved me to 13B, it was gone when I checked right away after.

And I tried it all to get it back - log out/in, repeat; tried website log in/out/in, repeat; deleted app, re-downloaded app, nada. Tried it all again after I landed in LIT, and again in the morning. But, my flight credits posted so I guess that is all that matters now.

Just weird and random. And personally not run into it before. Thanks for the links.
Doris