We're definitely looking into it, but it's unfortunately not as simple as "do this thing, result: game is broken". I wish it was. It's more like "do this thing, result: sometimes the game is broken", or more likely it's a different factor that is exacerbated by trying to kill the animals last, causing this repro strategy to have a higher success rate in occurring. Or perhaps there's 2 separate ways to cause the key to not drop, which could explain why some of the bug reports seem to have irreconcilable differences.
I've played a dozen games since the DLC released and I've seen it happen personally only once. Forcing a host migration did reset the match to a state where we could progress through the match. However, the other 11 matches did work without any issue. I'm wondering what I'm doing different if other people are getting it with a remarkably higher frequency.