This is a known bug. It happened in my old FC. A member promoted himself above his own rank and stole all our gil from the FC coffer. I immediately reported the person and a portion of our gil was recovered. I assume the other portion had already been spent as I was told they were unable to get it back. I was instructed by the gm to not allow promotions by any other rank and to keep the FC coffer set so only the FC leader can withdraw from it.