I just bought 2 TVs (UN65TU8000GXZD and UN43TU7000GXZD) and I'm facing problems to perform the screen mirroring using my Samsung S8 Plus and my iPhone 11.
The results are the same using both mobiles and TVs. I can find and connect, but the screens keep black and the sound works fine.
When I tried to use my old mobile Samsung S5 and my iPad Air 2, everything (screen and sound) works fine!
Additionally, I can perform normally the screen mirroring using Samsung S8 Plus and iPhone 11 in other models of Samsung TV. So it's not a mobile problems.
I have exactly the same problem. Screen mirror works with my older Samsung TV but not with new UE50TU800: sound on TV but with black screen.