Vengeance: disappearing act

You may now see that you have the option to pay 3 reward points for a "Vengeance Attack" when you lose a fight.

A lot of people have been waiting to test out this little feature. I clicked the button to buy a vengeance attack, and I did get full health, but it said I still lost the fight and then the option disappeared entirely. A couple of other players have also said they saw the option but had it disappear.