I have the exactly same problem on my galaxy note 8 after the last update of april, just get back to normal after leaving it alone and turned off for a few hours. Sometimes the problem returns for a few seconds.

Not only the note 9, my note 8 also had this problem with the last opdate on Android 9, but suddenly after leaving the device off for some hours the screen returned to normal, sometimes it returns but only for a few seconds.

I got the same problem in my Galaxy note 8, I even tried to roll back the software but no changes at all. After that i have just turned off the device for a few hours and then it came back to normal. Sometimes the screen do the same glitch again but ...