She's addicted to OM. As long as you don't interfere with her addiction, she'll be appease you. Maybe even through you a hug for being a good guy. If you confront, expose or otherwise interfere with her affair or try to make her feel bad for committing adultery, she'll be upset with you and punish you.
Also..realize YOU are not her focus of attention ~~~~OM is. Thus, she maybe she needs to tell OM she's talked to you and you are all fine with the affair (you told her she could do what she wants) or OM is nervous and your wife perceives that he is getting upset about you knowing about them and possibly spreading gossip about him and hurting his reputation. Your wife is being nice to you to help calm OM down and reassure OM that she has you under control.
Or...
Who knows. She's an active cheater...she could be in a good mood because OM broke up with his wife or girlfriend to be with her (supposedly) or OM had sex with her an hour before she got home and she's on cloud nine and unwilling to allow you to ruin the moment so she acts happy.
The point is...she's acting in her own self interest without real concern for you or your feelings whatsoever.
Were you too nice??? Nice is OK. It's better than yelling and being angry. Neither gets you very far towards recovering your marriage. Until the affair is over and no contact is established you are stuck, waiting for the affair to die. However, yelling and screaming actually make your situation worse and merely feed into her justifications and rationalizations. So being nice is OK...just don't overdo it by saying things like "go ahead and you are free to have all the extramarital sex you want" and perhaps "I am not your keeper, the cage door is open and you are free to leave anytime you want". Yours condones adultery, whereas the later just confirms that you are not her jail master...that she can pack a bag and go right then and there if she pleases.
The internet is 90% complaining and entitlement and I hate it because I deserve better!