There are a number of quests that have a "Fail if X before completion" anti-objective, which then show fail after successful completion. The solo Arachnophobia, Stealthy Repo (if you stick around to kill kobolds), etc etc.
(For new players, check quest experience ([x] key) to see if you in fact successfully completed or no.)
Seems a more widespread problem (and, one would hope, a more general fix).