BOAC
"Known" is possession of the relevant facts. Those facts were laid out in the BOI upon which they made their decisions.
Speculation that there may have been some fault which denied the Pilots the ability to control the aircraft immediately after waypoint change and then disappeared without trace is just that - SPECULATION.
There are people who believe that the Film "Capricorn One" (starring that fine example of American chivalry, OJ Simpson) was an exposition of the fallacy of the Moon Landing.
I do not believe that one either.