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.