Same thing happened to me. It's not actually broken. Since half points are awarded, (or actually subtracted), the counter rounds up. The quest is likely 99.5% completed. Another kill, and the purple tag should fall.

Same thing for me, had the Thorn quest bug and now this one...

I can't get any kills to count for this!

I'm having an issue where it isn't tracking my progress at all. I'm stuck at 0%. went on a 10 kill streak stoked to see my progress it had I checked it. it had gone from 1% to 0%. it's bad enough I have to give up my primary for an already terrible now nerfed exotic hand cannon but apparently the quest won't track at all for me. on top of that the requirement for this is ridiculous

Same for me got to 100% then nothing. Really bad glitch man. The quest is hard enough without this happening.

Try just talking the the girl that gave it to you. It happened to me when doing the quest that gives you the sword. I had all the materials and it showed a quest icon at the tower. But when I went there no one had anything. Talked to shaxx and he gave me the sword