Namely: if you swipe from the sides of the screen to go back/forward, you get a duplicate animation. If you swipe back from B to A, once the native (interactive) slide animation finishes, the page then jumps back to B and then does another (noninteractive) slide animation to A.
Fixing this would be as simple as disabling the page's slide animation, but whoever made this site didn't notice or didn't bother.
Apple does have some blame here. Disabling the page's slide animation wouldn't be a perfect solution since it wouldn't distinguish slide gestures from other ways to go back/forward (which ideally would still show an animation), like using the browser's back/forward buttons if accessing the site in a browser rather than as an app, or using a keyboard shortcut with an external keyboard. I don't think it's possible to easily detect which kind of gesture triggered navigation (in order to conditionally show the animation).
Another option would be to disable the native slide gesture entirely in favor of a fully JS-based one. In the past it wasn't even possible to disable it, but that has been fixed for years now [1]. However, it's hard to exactly replicate the native behavior.
Ideally there would be a more purpose-built interface to detect and customize the native swipe gesture.
[1] https://pqina.nl/blog/blocking-navigation-gestures-on-ios-13...