It looks like some sort of graphics corruption. I’m guessing in 5.6 the post processing volume which basically uses a texture to render effects such as bloom might be behaving strangely.
Try deleting the post processing volume and see if your viewport looks better. Maybe the shader cache in your project is damaged. Try deleting derived data cache and rerunning the project and see if rebuilt shaders don’t fix the problem. You can also try resetting your AMD drivers to default values.
Maybe 5.6 is more taxing than the previous version causing more stress on your card. How are your cards temps in 5.6? check with HWMonitor if you are not hitting some sort of heat limit while running the scene. You can also try limiting the FPS with the t.MaxFPS command and see if that changes anything.
In worse case scenario try switching graphics RHI to DX11 or Vulkan and see if it clears up.