There is one problem: the key does not drop.
It seems that there are two potential reasons that it does not drop:
A- a killed mob does not despawn and/or add to the kill count properly, or
B- a mob does not spawn properly resulting in a no kill situation.
Both result in a key not dropping.
There are likely multiple triggers for reason "A". I have played in many sessions of all melee classes during the weekend axe quest event, and ran into the glitch multiple times. It's not always an animal, this was happening before animals were in the game. (although, the weird sudden spawning of some animal groups that seem to show up out of nowhere may have something to do with reason B. Sometimes you can actually find a "killed" enemy that fails to despawn. It is usually invisible, but can be hit with attacks, which makes it visible for a time. They are not always the last enemy killed, we have found them in the first room of the zone before. The trigger is not exclusively an elementalist issue, however, I do wonder if being under the effect a CC mechanic....any CC... may have something to do with it. This has happened with many different mob types in my experience. Most often a red templar stalker/rogue thing, a ghost, one of those ice witch squealer things, or a little dinosaur thing, in my experience. (sorry i don't pay much attention to the actual mob names, but you know what i'm talking about if you've played much)
The main point is that there appears to be multiple trigger/variables/mechanics occurring that contribute to a mutual result, ergo the key glitch.
The solution could involve ironing out every one of those, (the hard way) or it could just take a well thought out change to the "key" mechanic itself (this seems easier to me, but i'm no developer, so I could be wrong)
The purpose of the key is to ensure that all the enemies are killed before entering the next zone, so people don't just run through it, which would, presumably, result in boss/completion farming...to many aggro-ed mobs on the board at once...etc. In other words, it would upset the system. Is there a way to do this that would sidestep the issue? Perhaps setting a "3 enemies or less, left in the zone" threshold for key-drop? That would just leave ironing out any potential spawn-in issues....IF...they involved more than 1 or 2 mobs. If a spawn in issue was only a single mob or two, the threshold setting would solve both issues. It would allow for a key-drop even in the presence of up to 3 glitched mobs total, regardless of the glitch type. It seems like it could be that simple, but again, I do speak from a point of programming ignorance, so...::shrug:: just a thought.