Safari 15.4 dropped prefixes for backface-visibility. That might be the cause of the fact that slides that should be hidden in 3D-space are still shown.

Browser:

• Safari 15.4 on desktop (Monterey, Big Sur and Catalina)
• Safari 15.4 on mobile (included with the latest iOS 15.4)

Example:
https://codepen.io/martino/pen/rNpLVmE

Go back and forth a bit: hidden slides pop up again. It seems to happen only before and after the second section, which has nested sections.

It happens in the main Reveal slider as well, when you switch it to ‘convex’.