Date & Time: June 1st, 11:15 AM CDT
Frequency: always
World name: Diabolos; Event server?
Character name: Katotsu Kilaandar
Monster name: Ser Adelphel; Ser Grinnaux; Ser Charibert
Class/Level: BLM66; PLD 61/62
Party or solo: party; 2
Area and coordinates: The Vault
Steps:
1. Run the Vault Undersized as a job that can acquire experience
2. defeat a boss
3. check for experience gain

My theory is the game flags completion of these fights to give experience, and the flag wasn't removed by Undersizing.