This has been happening since the feature pack went live on September 15. I’ve noticed that when attempting a number of activated skill challenges (Carnie Jeb and Klakka the Brain have been the most recent offenders for me), should you go into stealth and not immediately activate a stealth attack, the NPC you’re facing will exit combat for some unknown reason. You can still damage them when emerging from stealth, but they won’t fight back, beating them won’t provide a skill point and as soon as they reach their point of origin, they will heal completely, forcing you to restart the challenge.
And when I say “immediately”, I’m not exaggerating. You don’t even have enough time to reposition for a backstab, which takes all of a second and a half. That’s why I’m certain this is a bug.