Enemy disappears while fighting

[dalinrhoze]

I was fighting siren for the mission, after getting her down to about 1/4 hp she simply vanished, this was very disappointing as I had spent about 5-10 minutes fighting her already, tried to click on the ??? After it happened, and the game gave me a wait message, waited a bit longer and clicked again, and the fight started all over (at which point I just warped and exited the game, SO flustered)

[Halley]

There's a time limit to keep people from taking 5 hours to kill it, you ran out of time