I too see this exact problem. It does not come when we make calls via Bluetooth. Only when making calls directly from phones.
Since you full screen bezel less display, may be we are seeing this. I'm hoping it is not an issue.
It is not an issue, it is normal. That is exactly what I pointed out in my previous post. If you have BT headset connected and make a call, you won't see it.
Guys, please believe other posters that explained perfectly that it's just a proximity sensor doing its job. As we all seem to want nothing but screen, numerous sensors are now hidden right under the screen. This is noting to be worried about, it's how Samsung have implemented it. A pretty cool tech IMO.
Everyone, this is not a bug nor screen defect. This is an indicator for the proximity sensor. The S10 does the same, due to the location of the module.
If you cover the indicator the screen turns off. Why? Well that's what a proximity sensor is for. It detects when it's covered so that it can turn off the screen during calls.