As I understand it, all resources a clan member gathers is stored on the player. Citadel data can be pretty glitchy (e.g. being unable to set new targets with the "0000007" bug or w/e, visitors not being counted when they enter, inconsistencies of who can cap, receive bonus xp from quartermaster, etc.) Somehow, the partial cap didn't clear when the one player left their old clan. It probably won't reset until Shattered Moon's tick next Tuesday is my guess.
Did they try leaving and re-entering yet?
22-Sep-2016 19:15:18