Hello,
This is a bug we found a while ago and had trouble reproducing it.
We have a UI customization comprised of simple CSS and HTML that adds navigation links to the header. If a user starts on the admin section of the website and then travels to the frontend by clicking on the logo this customization does not render.
It should be noted that this customization does not render on the admin normally so perhaps Discourse is saving this state when it routes to the frontend?