Hi. I think I found a solution.
The issue was also reported on reddit, so I’m guessing that the issue arises when your device is connected to an external monitor.
I can’t really tell if the issue is coming from Netflix or Safari, but I also use a MacBook Pro with an external monitor (LG Ultrafine 4K).
So the solution is—instead of entering the full screen mode by clicking the full screen icon in the lower right corner on Netflix or by double-clicking the video you’re watching—go to the menu bar on Safari. Then, select “View” and select “Enter Full Screen.” Voila.
In other words, Safari menu bar > View > Enter Full Screen.
Thanks.