Move your dial when "DAWN" is shown on top will give you 1st sunlight BEFORE sunrise. You should be able to start seeing thing with some light coming up outdoor.
Move your dial when "SUNRISE" is shown on top will give you when you actually can see sun.
The Astronomy face seems to only show where the sunlight directly hits the Earth, i.e.: it doesn't simulate the atmospheric scattering effects that lead to "twilight". While this might be a nice improvement, I guess it's not really a bug or defect with the Astronomy face.
But that's a good idea to use the Time Travel feature to check the Astronomy face at sunrise! In beta 5, it does indeed look correct (at least for London at 05:34 today).