Thanks @Paul-Harris - for some context, f.lux v4 avoids constantly looking for color changes and tries only to "fix" what other programs do in a few specific ways, so you're seeing what's expected, except that f.lux should definitely be detecting any new screen that makes your desktop bigger, so that's a bug.
That list currently includes: plugging in a new device (which seems like it's not working here), returning from sleep, and switching apps (like alt-tab).
When you plugin a device, does it look right for a bit and then reset later? Or does the tint instantly disappear?
@onotole f.lux actually modifies this setting for activation at sunset. Then I assume it sets the "dark theme" back to unchecked, once sunrise happens. It's probably enabled by default in f.lux, so look in the settings there first.
@raj.seshadri You need to have the app, i.e. Safari, running, and in the foreground, to see the option to enable or disable f.lux for it. For example if you're in the Finder, you won't see the option for Safari. Does that help?