Not lost, withheld. Every year when they release information from the national archives that is twenty years old (used to be thirty years) some documents are withheld for a longer period of time, usually for security reasons or to avoid embarrassing those still living. Not a true conspiracy because it is government policy. If they with held them for another twenty years then I would think there was a cover up but by then most of those involved would be dead. That's how it works!
Looks dodgy, is dodgy, but everyone knows it happens.