Unusual C+ upgrade seat assignment
 

On an upcoming trip the MSP-SAT segment showed only C- middles and no C+ available and I had no seat assigned. I figured that a C- assigned at the gate was no worse than picking one now. Finally decided to ask for C+ upgrade since a middle C+ is better than a middle C-. Surprisingly got an immediate upgrade and was able to choose an aisle. Felt pretty good until I checked back and found I now had no seat, confirmed C+ upgrade but only a C+ middle available to pick. I will hold off on seat selection hoping for something better at the gate, worst case being just another C+ middle. Just wondering how I got here but it does show how people on the standby list can be confirmed into either FC or C+.

Could the upgraded aisle seat you picked have been taken back for someone with special needs?

Not likely but possible. It was in the third row of C+ so not a bulkhead. The removal was done within a few hours, Another possibility is if someone was monitoring seats, saw an aisle and we grabbed it "at the same time" leaving me seatless.

Curious if this was a business trip booked via an online system. For several months I've experienced a glitch, just with my corporate bookings. Not every trip but several times.
For work, I can only book coach and I need to use Concur software. I typically get upgraded to C+ right away but often my seat assignment vanishes soon thereafter. I retain the upgrade but my seat is unassigned. I try to monitor seats and grab the same aisle or another one back, but sometimes it's too late and there are only middles left. Yesterday I got upgraded to F (non-bulkhead), then was unassigned, and then reassigned to a window though my preference is an aisle and there were several available including the one I had.
Guessing I'm not the only one experiencing this. It's a hassle to have to review all your trips on a daily basis. Hope DL IT figures it out.

This is the first time it happened to me. At reservation time C+ showed full on the seat map (I know about the 2 held back for the gate) and for some reason was not assigned one of the C- middle seats available. After a few days opted to request the C+ upgrade (not automatic for me) and the above played out. Turns our it was on MSP closed day so I never took that flight to find out what would happen.