This bug occurs toward the end of Broken Citadel during the objective Open the Gate. I would get the key, like usual, and the usual infinite waves of endermites would spawn, but I would take damage and an endermite would take the key. However, as soon as the endermite dies, the key disappears with it, instead of it running around, waiting for another player, or mob, to pick it up.

Normally, if the key despawns, a new key is spawned in the same area the player first finds the key. Not in this case, however. In this case, a new key didn't spawn, and there was no trace of the previous key, as I didn't hear the audio the Key Golem makes. This caused me to leave the level, as progress was rendered impossible, as the key is required to open the gate.