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.
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).
