The actor's off screen behavior overshadows his on screen charactor. And it doesn't have to be bad behavior. I recently watched a rendition of Joan of Arc. In a story about a Catholic (Christian) saint, God was played by a Jewish actor. Jews don't believe in Christ, let alone saints. So his performance was overshadowed by him being Jewish. He was therefor, unbelievable in the role he played. Is he a good actor? Yes. Was he a good actor in this role? No, because his off screen persona conflicted with his on screen role. The same is true of many of these actors, if they played the role of socialists or communists, they would be believable, but they don't. They try to play Americans who love their country, but their off screen persona conflicts with that.
If an actor can not convince the people watching him that he is who he is playing, then he is not a good actor.
This can happen...but it doesn't happen for me. Lucky me.
I'm not saying I don't notice if an actor is miscast, if he can't properly portray his character. But I don't have to know anything about his personal life to make the judgement...and conversely knowledge of his personal life doesn't affect that judgement. I've seen perfectly straight actors portray wonderful gays...and I'm told that Clint Eastwood is a non-smoker in real life.