In Juno Beta 2 night mode worked as intended. I've performed a clean installation of the stable release, now night mode does not respond at all. Using an Inspiron 3558. I can see the night mode icon appear in the task bar, have tried rebooting and toggling on/off - nothing seems to kick it on.
I installed Juno yesterday, and night light was working during installation from an USB, but later it did not show up, and I experienced the same behavior. And it's weird, but just setting the schedule to manual and changing the time made it show up and work perfectly. Hope it helps