Please ask this question to all NOTE 9 users. After the software update in March 2020. all Samsung NOTE 9 user has Discoloration of Screen and Samsung is not taking any responsibility.
NOTE 9 A BIG SCAM from Samsung and I guess it will be for all NOTE users. Be aware when you are buying NOTE 20.
This may be a hardware issue. Try going to a certified Samsung Service Center to get the best care for your device and the issue.
Hope this helps!