Finally, an opportunity to fix the incorrect behavior of the game (e.g. an instant brightness increase at dawn) when changing parameters surface.dawn or surface.dusk - has been found!
So it was game bug for sure - so I posted report at https://forums.factorio.com/viewtopic.php?f=7&t=88585

Mod bug is that mod does not know how to correctly restore the default values to the game. So after disable/revove mod, the bug continues.

Thank you for a great mod!

Thanks :)

There was the request about the button in the mod setting, for example "set to default". https://forums.factorio.com/viewtopic.php?f=28&t=78880

Or change day properties simultaneously: https://forums.factorio.com/viewtopic.php?f=28&t=68495

hey avel,
thanks for the hint but until it is fixed in 1.1, can you or darkfrei please also tell me (and others who haved tried this mod and want their ingame brightness changes back to normal) WHAT THE DEFAULTS ARE and how to restore them? pretty please? you folks may have found a bug, but as far as i am concerned, THIS MOD broke my game for the time being by changing the parameters permanently... :/
basically, i believe a mod should never make changes that will remain even after removing it! but hey, what do i know, right?! also: why even publish a mod that produces this weird behavior, darkfrei? if your modding skills correspond to the quality of your english, i will steer clear of your mods from here on out...!

/c game.player.surface.min_brightness= 0.15

and

/c 
game.player.surface.dusk= 0.25
game.player.surface.evening= 0.45
game.player.surface.morning= 0.55
game.player.surface.dawn= 0.75

There was no problem with this mod until devs broke their side somehow.

ah, got it.. thanks for the commands and explanation... have a good one. :P oh, and maybe, add a short warning to the mod description for the time until they have hopefully fixed it in 1.1?