Quest isn’t bugged. The time requirement isn’t the only requirement. You also need another fireteam member who is either:
1. Also working on quest.
or
2. Has Rat King gun equipped (not simply held in inventory) at time boss is defeated.
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This is 100% truth. No one really knew this because if you are a day one player most people all had the quest around the same time. It's newer players (or secondary/third characters) that had to find this out the hard way.