Thank you for your reply, Herf.
My monitor is a Dell U2417H, which I've already "hardware dimmed" as much as possible, but it's not enough, hence my reliance on f.lux for some additional dimming.
I understand your design choice. Allow me to suggest the following future features :
The return to full brightness could remain standard, but at least this should be a progressive return. (Currently, the return to full brightness is immediate, therefore brutally bright).
An option for permanently locking the dimming settings could be made available in the settings. It'd be an opt-in option, hence not activated as a standard.