Thanks for posting about this. I did some testing on this end and the site is working as expected in Webflow.
I also tested on iPhone XS and XS MAX using both Safari and Chrome, but was unable to see any horizontal scrolling.
One trick you can use here that may help though, is to set each main section of your site to overflow:hidden. This will ensure content isn’t falling outside of those boundaries and should help prevent any horizontal scrolling.