Hi,
This is the bounce that you get on ios (and on my imac) when you scroll to the bottom or the top of the page there is a bounce effect where the background of the body shows and then slides back. It’s supposed to be a feature but does mess with some designs.
There is a js fix for it.= if it worries you but it generally happens on all sites and is there to provide feedback that you have reached the end of the scroll.