I've experienced the same problem, but in MSP airport. The gate agent waited for boarding to start. Once group E started rolling, she started to pull passengers from the "awaiting seat" list and put them in first class. There were 4+ seats available and no one from the upgrade list got move to first class. I witnessed this twice in two separate occasions.

The second time this happened, I went up to the gate agent and ask if i could be upgraded to first class. She straight up lied that first class was full which I knew wasn't true. I had screen shots of the seat map, upgrade list, and awaiting seat list with time stamps, but Alaska Airlines didn't want them. AS just said they will investigate.