Since Mate 20 series, all of the Pro model comes with a notched display. The notch is to fit in all of the front facing camera and 3D facial recognition sensors. It seems like this design style will be used on the upcoming Mate 40 series too!
According to @RODENT950, the Mate 40 pro notch will contains a 3D facial recognition sensor (Face ID sensor) from his latest tweets.
Besides, the Mate 40 Pro will feature display at 90Hz which is on par with P40 Pro.
Regardless, the Mate 40 series will be launching by Sept to October this year. Stay tuned.