Zoning seems to blank out the quest reward window if the window is closed before zoning. Both /reward and the button from the EQ menu result in the "you do not currently have any pending rewards." message. Camping to character select and back fixes it. I'm not willing to do a whole lot more testing in case they decide to disappear permanently, but I've reproduced it a bunch of times zoning around guild hall/sunrise hills/plane of knowledge.