Haven't had that experience on AS (fortunately), but remember it years ago on other airlines. I can understand if it's within the boarding cutoff that your seat could be given away (same with non-upgraded seats), but otherwise was she really suggesting that you needed to camp out at the gate an hour before the flight?

My other annoyance with gate upgrades is that it inevitably breaks the boarding pass on my phone, even after it updates to show the upgraded seat. I guess a minor detail in the black hole that is Alaska IT, but still frustrating.