UPDATE: I have been informed by Squarespace that this feature has been intentionally REMOVED :(
@jgouger, @Melian, @eschafer, @CarlaJ, @tuanphan, @BarbaraD
I have the same issue and went a little crazy before realising this is infact a Squarespace fault. I reported this Friday (21st) and it was sent to their engineers.
I left my details - not yet heard anything back. I have a few sites and it only effects the most recent one, it also happens if I start a new trial site. The prev/next pagination is hidden with no way of turning it back on. Really annoying as crucial feature in an online store.