First of all, thank you very much for your reply and for offering to look into the possible cause.
The site where this issue is occurring isn’t live yet. We’re still in the process of building it.
But there is a development environment where I can set up access for you.
The site is currently protected from external access via .htaccess.
So I could give you the username and password for the .htaccess file so you can take a look at the site.
I’ve already checked to see if I can do something with CSS, but I think it’s actually a problem with the header JS being re-initialized.