Date & Time: Noticed at 11AM JST, 15MAR18
Frequency: Just the once.
World name: Gilgamesh
Character name: Serge Ironbreaker
Class/Level: 70PLD
Party or solo: Party
Area and coordinates: Forbidden Land, Eureka

Steps:
1. I had attained Lv7 in Eureka.
2. Was not in Kugane or Eureka during the maintenance.
3. Have not AFK'd or logged out in Eureka.
4. Went back into Eureka after being AFK at my in-game house for 10h or so, noticed I had lost two levels.