If you’re asking about in the Beta, you might want to ask over there. In the last official release radar was not updating in real time. You had to change range or make some other change to get it to update.
But when radar was updating correctly in previous releases, there wasn’t a visible scan line. You could see where the radar was updating by watching the scan update the screen as it went back and forth, but even then there wasn’t a visible scan line like you’re describing.
At least on the PC version of the beta, the radar is back to updating as it should, but there’s no line.
Regards