Could you please share your published site and read only link, here’s how to share it, We can then take a look at your settings to see where the issue lies.
It seems that it’s to do with your custom code on your settings page.
If you remove overflow from the Fixed Element temporarily, when you view the mobile version in the designer you can see the issue happen.
If you remove the code from your project settings, the issue no longer occurs. Unfortunately we don’t offer support for custom code, and I’m not knowledgable enough to pinpoint the issue so I’ve changed the forum post category in case anyone can help out.