If someone catches a shiny Pokemon and saves, then the shininess is stored in the save file, and no amount of soft-resetting or rebooting can change that. "Soft resetting for shinies" usually means encountering a certain wild Pokemon, not saving if it wasn't shiny, and then soft resetting. Because the properties of the Pokemon weren't saved, it can change when the player soft resets. I hope that, if you had a misconception, this clears it up.