Staying quiet when you know the truth isn’t neutral. It just picks a side quietly.

Silence feels like standing outside a disagreement and functions as standing inside it. Where one version is wrong and you are the person who knows, saying nothing lets the wrong version win by default, and whoever is harmed cannot tell discretion apart from agreement. Nobody will accuse you of lying, which is what makes the option so attractive.

You do not need a confrontation, only a plain sentence: that is not what happened, and I was there. Say it once, to the person who matters, in an ordinary voice. If you only know part of it, say that part, which is usually enough to slow things down.