Downgraded from First 15 minutes before boarding. Anyone seen this?

I bought a U fare First Class ticket and used an upgrade certificate to lock in a First seat. Checked in online right when it opened.

Before boarding, I tried to check my carry on, but the agent said I missed the cutoff by a few minutes so I kept the bag. While going through security, I got a notification that my seat was reassigned to economy, then again to premium economy. At the same time, I noticed another passenger from the upgrade list had been moved into my original First seat.

I asked the gate agent what happened and they couldn’t really explain it. Eventually they said they weren’t sure who “should” have the seat, but since the other passenger had already boarded, I should just take the seat shown on my pass.

They apologized and told me to contact customer service after landing. One agent mentioned that my attempt to check the bag after the cutoff might have caused the seat to be released, but that sounds odd to me.

Has anyone had this happen before? What’s reasonable to ask for from Alaska customer service in a case like this?