I found there's another bugged quest (other than the "Sharandar rewards") named "Assassin Search" in Rothe Valley area. As you can see the quest says "throughout Rothe Valley". However, at the count of 2 the number ceases to increase, altough I must have killed at least 30 of them. I abandoned and took the quest several times, hoping it would unbug but it didn't. Every time the counter reaches 2, the game stops to recognize next kills and the quest can't be completed.
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I am having the same problem now. Killed plenty of them. The only difference is that my counter is stuck at 5 kills out of 6. Maybe if your companion does the actual kill it doesn't count? Just a guess though.
Neverwinter Census 2017
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