It comes on at sunset and on at sunrise. I set it to manual and it doesn't enable dark mode still. It turns night shift on properly just fine on either setting (it shows it as on until sunrise or on until 7 AM in Notification Center)
Dark mode is only possible manually for me no matter what. It started last week and has been that way since. the dynamic wallpaper works fine, but the only way to make 'auto' dark work is to go to light mode and back to auto and it changes. it will actually go light mode properly on its own just not dark. It was 9 PM last night and it was still in light mode. night shift was on, and my dynamic wallpaper was dark.
Manual is the only way it works. Shame, it was a nice feature while it lasted.