It’s not a “fake” as in counterfeit. The problem is that is was put up for auction as a legitimate da Vinci painted by the man himself and the truth is a bit more complicated.
Experts kept looking into it and they came to the conclusion that while it was indeed made in da Vinci’s studio, it was actually painted by one or more of his assistants who are masters in their own right. Leonardo only provided some input, guidance and/or perhaps some strokes here and there at best.
It’s still a valuable painting, so what’s the problem? If it wasn’t painted by Leonardo himself it’s not worth anywhere close to $450 million. $10-20 million or a little more, yes, but nowhere close to what MBS paid.