UPD. I checked your site until full loading. At this moment, the browser toolbar panel is static. After page full loading, I saw the problem - it is associated with a floating browser toolbar. When it appears Header disappears. Nevertheless, there was no such problem in Helix Ultimate 1.
For the developers team, you can reproduce the problem on the desktop. To do this, you need to cause a shock effect on the page. If you click F 12 To invoke Dev Tools Header disappears, just like on mobile device. After closing the Dev Tools Header begins to work not correctly until page refresh.