Goshdarnit
Flexible tickets (e.g. F and J, even Y) can and do no show with impunity.
Some companies block book seats in anticipation of travel needs and then no show them.
Of course airlines will overbook F, based on predicitve modelling of yields, but but it is customer relations disaster when they bump a passenger.
Were I RMAC, I would be absolutely furious and would be seeking major compensation as well as the re-routing.