My grandfather was a bomber pilot in the Korean War in the Black Sheep. On his last day alive, he told me a story that had bothered him since the war. He had been flying in formation with a few other bombers, regular run heading north. It was overcast, and out of nowhere one of the US bombers banked West and dived below the clouds. They lost track of the plane. Highly unusual, no radio reply. Upon return landing they were briefed to forget about the encounter, tell no one about the plane that disappeared. Is there any way to verify this story? Are there any notable disappearances from the Black Sheep squadron?
None of the publicly-listed losses for VMF-214 seem to fit. It might be worth combing through the unit's Historical Diary to see if there's any reference to this event. If the aircraft was from another squadron, then pinpointing the event becomes much more difficult, given the hundreds of aircraft lost by the U.S. in the Korean War.
I will say that defection seems like an unlikely explanation. To begin with, there are no known cases of pilot defection during the Korean War -- one North Korea pilot defected shortly before the war began in 1950 and Noh Kum-Sok famously defected with a MiG-15 shortly after the armistice in 1953. An American pilot defecting to the North Koreans or Chinese would have been a major propaganda coup for the Communists, so it's almost certainly would have been widely-publicized had this happened.
It's possible the pilot's oxygen system failed and he became hypoxic and lost control of the aircraft or tried to dive to get into thicker air.