I acquired recently a D850. A great camera.
One of my test was to take a picture in Speed priority mode, Auto ISO. Using a Nikkor 24-70mm F2.8
Setting for Speed 1/160 with F2.8 aperture.
When taking this picture using the viewfinder, it came up with an ISO of 320
When taking this picture using the Live View, using the same speed and aperture, the ISO came up to 900 and the picture was much better exposed than in the first case.
So why such difference whether using Viewfinder or Liveview ?