
Pomegranate liqueur is a fruit liqueur of pomegranate juice, sweetened, on a spirit base.
Pomegranate with alcohol and sugar, so a rebuild needs all three: a syrup with a neutral spirit in roughly equal parts lands on the strength and the sweetness. The tannin under the sweetness is what a plain syrup lacks.
Grenadine
Grenadine is a non-alcoholic pomegranate-and-sugar syrup — a sweetener, not a spirit. Pour it in place of the liqueur and the drink loses several degrees and gains sugar, so cut the rest of the sweetening and make the strength up elsewhere.
Pomegranate in a bottle is not the same thing as grenadine, and the difference is not only the alcohol. Grenadine is a syrup: sugar first, fruit second, and in many bottles no pomegranate at all. The liqueur is built on the fruit and carries its astringency — the tannin in the seeds and membranes — which is what stops it reading as sweet-red and nothing else. That astringency is why it works in a stirred drink where grenadine would only sweeten one.