Originally Posted by
GagaPilot
Sounds to me like a DL IT glitch. If an F seat was available, and you were a confirmed displaced F passenger, an agent should have been able to move you to F if a seat was available. Did you ask?
And for the OP’s question, most likely a displaced F passenger as well.
My understanding is that it's not unheard of for GAs to forget that it's possible someone on the confirmed-but-awaiting-seat-assignment list to be booked in F. Such a GA will clear upgrades into F/C+, then assign seats to confirmed-but-awaiting-seat-assignment, and then clear standbys. The DL app and gate screens combine the standby list and awaiting assignment list.
I personally have a near-100% success rate at getting a redcoat to correct things by unupgrading someone who hadn't boarded, though it generally means missing a PDB.