We’re using the most recent versions of both WP and your theme. In doing compatibility tests, we noticed that your theme has rendering issues on iPhone devices below 6 (i.e. iPhone5 and below).
We can confirm that this is, indeed, a theme issue because we ran a multitude of internal tests, but noticed that your theme demo itself http://janstudio.net/gecko/fashion/ (see below) replicates the issue.
It would seem that either the theme has an issue with a) ‘full-height’ parallax backgrounds b) the usage of ‘flex’ display types isn’t compatible with iPhone 5 and below devices. We tried various CSS child-theme overwrites (i.e. max-height:100%), but these did not work. In other words, on iPhone 5(s) and below, full-height blocks are sized incorrectly.