Based on your description it is most likely that for some reason the game did not record correctly the fact that you restealthed during combat (for example you failed to actually get out of sight from the enemy).
It would work that way if I hadn't had the chance to end combat mode during the fight. But I did.
After that I dawdled under that camera on the wall, while Pulverizer was recharging his shield and waited for me to appear. Actually I also don't know why he didn't run around the arena trying to find me. Thast would be logical.
I waited for the cooldown on Aimed Shot to pass and shot him from the stealth. Similar to what I did with other char. Only it worked last time and I got Apparition, but this time Phantom was... replaced.