Until last week the Emerald Lair room did also grant an Epic Vault Key as a reward.
Today I catched this change (see above): aaaaaand it’s gone.
So be aware that the Epic Vault Key was obviously removed as a reward before spending your precious lantern on that room.
Nevertheless: Have fun!
-- Hunk
what the hell? come on devs, hoping that this is just a mistake, but looks intended.
And what happened after the battle?
Probably just visual. You know, the 4th reward can usually be hidden, especially when you’ve played other game modes that use similar rooms (delves & tower of doom).
I doubt that’s a “visual problem”:
All my drivers are up-to date, I have no graphic errors or distortions in the game, and the Epic Vault Keys a perfectly visible in my wallet or when fighting a Gnome Vault.
If the 4th reward is hidden, it must be a bug in the game.
I did not fight this room, I will try tomorrow after reset and hope that the devs will enable the 4th reward again 
You normally don’t see the epic vault key on that image before the battle. Are you sure you don’t get it afterwards?
It’s a visual problem. It happens to me every week with a different Bosses. I think after some X number of battles, the last reward of the room no shows. Idk why, but this problem is there already for a very long time.
If you will close the game and open it again, the reward will be visible
This happens to me all the time. I keep a record of what to expect from each boss, just in case.
You always get the key, and whatever other rewards are not shown.
But, if you’re worried, be ready to screenshot the reward screen, just in case.
It’s one of many, many, many bugs that have been pointed out, I’m pretty sure, and we’re still waiting for any fix. Since it’s just visual, I doubt it’s high priority, but it should be medium, since (as you say) it affects whether people want to fight that battle and could affect how people use lanterns.
