I've found that to be true.
My dad was an Army Major with the Special Forces in Vietnam when I was a kid. On furloughs home to Okinawa, he would always be sort of spooky, and Mom made us kids walk on raw eggs around him.
That was in the mid-sixties. To this very day, he's never offered a word to anyone in the family about what he did in Nam, although he freely reminisces about our family's many moves, and his other deployments.
Nam is like a redacted page of his personal history, and none of us would even think of pressuring him to talk about it. Even after all these years.
My grandfather was US Army Infantry in France 1917-1918. He was in the trenches until the armistice. Never spoke a word about it to anyone period. Barely acknowledged that he had been in the army. Best to let those memories die and stay that way.