I got my stickers back on my 16 Pro, and I can add new ones. Not sure when or how it was fixed. I'm currently on the 18.2 RC but I'm pretty sure it was fixed on the previous beta. I didn't have to to factory reset/restore my iPhone or anything, they just magically appeared again.
I heard it was fixed on some iOS 18.2 betas. I just installed 18.2RC and reactivated the stickers again. It was empty. I created a new one and all my old ones appeared again.
I've seen a few reports on Reddit and forums where people had the same issue over the course of the iOS 18 beta. Anyone knows of a fix? This is on 18.0