So what it sounds like what your seeing is the proximity sensor. Since the sensor is behind the screen, during calls, etc., you will see a single pixel lighting up to the right of the selfie cam. This is completely normal. If your on a call and place your finger over that pixel, you should see the screen go dark.
I hope this info helped in any way. Have a great day.
This is totally normal. No need to send your device in for repair. That flashing pixel you see is just the proximity sensor on the device. It is now embedded into the display and flashing when a call is in progress or any type of whatsapp or skype call or voice message is being played. Like @MB-837 mentioned below if you place your finger over it, it will dim out the display. This is the same case for Note 10 & S10 series as well. No cause to worry all is well
I am having the same issue with my S20, but its not specific to just on call, it also seems to happen with the adaptive brightness in dim light. As I have already replaced the phone once before, I just want to confirm that there is no further issue as its happens regardless of apps or on calls.