Lando was looking out for the entire population of Cloud City.
If he had called the evacuation too early there would have been a blood bath, if he had warned Han there would have been a blood bath. He tried to negotiate with a Sith Lord.
This never goes well.
Leia should have known this but her emotions got the better of her.
In this regard having her Vader's daughter makes perfect sense in retrospect, much more so than Luke being his son :
