Hello,
This happens because on mobile the header area has a limited width, so which is why it pushes the icon off-screen.
To fix it, please try adjusting the element placement inside the Header Builder:
* Move the HTML Block to the Header Top Bar or the Header Bottom Bar in the mobile header
* After that, check again on the mobile preview
Since the mobile header has a different layout structure, small spacing changes can affect alignment.
Best Regards,