Do you have evidence that they were?

Sounds to me like GA processed the standby list, starting with OP, and OP cleared onto the flight before anyone else. Someone (we speculate NRSA) was standing by for the F cabin, and as there was an empty seat when the GA got to them, they got it. OP was not standing by for the F cabin.

While NRSA are listed on the passenger-visible "upgrade list", I don't think they are actually in line for an upgrade. They clear straight into the F cabin if there is room, just like a paid F standby.

You are proposing that the GA should switch to clearing CPUs immediately after clearing standby each passenger into Y, which is not what I think happens. They process upgrades. Then they process the entire standby list (F passengers (rev or nonrev) clear into F, Y passengers (rev or nonrev) clear into Y). Then maybe if there is time they will process some more CPUs if there are empty F seats.

GA or others with more info can correct me if I'm wrong here.