Time: 12/2/2015 (I think?)
Frequency: Once (Did not reproduce)
World: Exodus
Character Name: Nefarri Malzahar
Retainer Name: None
NPC Name: None
Monster Name: None
Class/Level: Might have been BTN or MIN (level 60)
Party or Solo: Solo
Race: Miq'ote
In-Game Time: Don't recall
Area and Coordinates: Golden Saucer, Minion Square
Housing: None
Steps:

1. I completed all challenges for the Lords of Verminion achievement: "Challenge Accepted."
2. I received on-screen notification of completion of the achievement.
3. I searched my inventory for the Clockwork Twintania minion (achievement reward). It wasn't there.
4. I checked my minion collection but still could not find Clockwork Twintania.
5. I checked character achievements to verify I got the achievement. It says I did.
6. The Clockwork Twintania minion is nowhere to be found.