I checked on your website, it is not done because of the iOS device, it is done on every device when in mobile mode (even desktop with the console view to test on small devices).
The problem is either with the plugin you use to compress CSS into one file or with the styling in the theme or something called ‘vc_grid-filter’. There is a broken responsive block that when compressed into one file, flattens a lot of other CSS belonging to other plugins into media query for a screen width of 768px and more, so all the styling for my plugin has been caught into that (and other stuff on your website, a lot of things are most likely broken), so styling for my plugin is used only on screens bigger than 768px in width.
You need to either find and fix that broken media query or figure out a different way to compress CSS without scripts merging this way.