Right which is standard procedure. You cannot buy a cheap round trip ticket using Q or whatever then refund the part you don't want to get a one-way that they'd never have sold you to begin with. So they are charging you accordingly. I think "no longer available" means had you tried this as one-way it would not have been available then either. So you are paying the one way fare. Q is very likely only a round-trip fare basis.

These airlines, and it's not this airline, but all of them, do not have revenue management agents born yesterday.