Sliding effect works... then doesn't (Safari)

Using the example (https://alvarotrigo.com/pagePiling/) in Safari I can scroll to the bottom just fine, and up once usually, but if I scroll down again (or stop on the 3rd and go back down to the 4th, or something like that) the scrolling up animations don't work anymore, only the scrolling down ones. There is a delay before showing the next page, so I suspect the animation is still occurring and this concerns z-index. I was not able to replicate the issue in any other browsers.