After a new clean installation of Juno I tried to enable Night light with the automatic sunrise/sunset timing. This did not work as expected. What did happen was, that night light kicked in only at the (previously) designated manual times. You can try to switch to manual and change the times, and this setting will stick even after changeing the switch to "sunrise/sunset".
There is a similar problem with Ubuntu that I know of - if a user selects the same option in gnome-settings, but does not enable Location services (in a separate settings panel), the functionality won't work. Unfortunately enabling location services in elementary switchboard doesn't solve this issue in eOS. This should be addressed by the devs.