On my flights if there is an open first class seat, any D6 will be in it. It is captains discretion to move them up and we on the j/s committee have constantly reminded the pilots of this fact. The problem is that 1) many times D6s don’t come up to introduce themselves if they already have a seat, 2) many captains don’t check the standby list so they don’t even realize they have a D6 on board, and 3) even if they do realize they’re on board, they can’t be bothered and /or don’t want to engage with the #1 FA. Sad.
Agents can absolutely issue a first class seat to a D6 if that’s the last seat left. But some agents don’t realize this.
Sorry this happened to you.