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.