Looks to me like the background images just aren’t working with the aspect ratio of the iPhone.
Background images are a pain to get to work perfectly with mobile screens depending on their aspect ratio. Sometimes it’s necessary to set custom mobile background images to the sections on mobile.
iPhone/iOS doesn’t support fixed background images either, which are what you’re using for the parallax effect.
Another option for a site like this is to disable the mobile mode: https://wordpress.org/plugins/generate-disable-mobile/ – it may look better as it should keep the aspect ratios as they should be.
Let me know 🙂