When I set dark mode after sunset and wait until sunset it doesn't work. Location services are enabled, time zone settings are correct and night light works after sunset. Why dark mode not working same as night light? On my second laptop with ZorinOS17 it works fine.
Where You set that up? In Zorin Appearance? If yes please post a Screenshot of the Settings.
The Night Mode in the Gnome Settings doesn't affect dark or light Mode it only make warmer Display Tone for unstress the Eyes.