I think this has been brought up before, but I don't know if it was ever noted as a consistent bug. It seems that, at least in tournament games, the game remains in vacation mode even if the user's vacation period has expired.
Example game: https://online-go.com/game/7326647. YelloFresko is listed as being on vacation, but his time would have expired around the 6th and his profile no longer lists him as being on vacation. This has happened with consistency.