
Sambuca is an Italian anise-flavored liqueur, sweetened and colorless in its classic form.
Anise with a lot of sugar, and the family splits into sweet and dry. A dry bottle needs syrup added; the louche is the tell that the anise is real either way.
Family and role match, but sambuca is sweeter: around 350 g of sugar per liter is its floor. Substitute the drier bottle and the drink needs its syrup back. Swapping back, the drink wants its syrup returned.
Dry, with licorice over the anise: it gives the same louche with water and none of the sweetness, so the syrup has to come back.
Anethole is what makes sambuca louche, and it is also what makes it crystallize: in a cold room the bottle can throw a white haze or fine needles. That is the anise coming out of solution, not spoilage, and it goes back in at room temperature. Keep the bottle out of direct light.