Munn isn't perfect. And she filmed with Singer before MeToo and TimesUp happened. Even now she is at risk of being blacklisted for "problematic" behavior such as calling out a director who hired a registered sex offender, the very thing that Hollywood keeps failing to address (amongst the other -ism's it hasn't fixed).
So working with Singer was probably a calculated risk to her. He was questionable but she's an actor, they need to work to make money and this was a big deal movie so I can see why she signed on.
In this case though, she was not told about this guy's past even though Black knew it and hid it from everyone. Once she learned of it she was supposed to what, not call it out and be a hypocrite for knowing his past and not saying anything?
No matter what she said or did not say, someone was going to use this against her. She might as well stand by her convictions on this and take the heat rather than stay silent and accept that Hollywood won't change on its own unless people call it out every time.