Same thing happened to me. they bug out and disappear. I followed the advice of the others on here and just stayed near a spawn point for the minion/wolves. when a wolf appeared I started attacking it and it died before the minion could be killed. I also killed the vines right away as they never respawn (at least for me). the southern most minion/wolf stopped appearing after Jory and Kas disappeared though, so I just moved to the next wolf. Took awhile but was able to kill the boss with a Staff Mesmer and my clones.