<link href="style.css" rel="stylesheet">
Because you’re using relative URLs.
When you are on example.com it will load example.com/style.css
When you are on example.com/test it will load example.com/test/style.css etc etc
As you move around the site, it will change where it’s looking. None of your pages ever referenced your style.css, it’s probably returning an empty 404 page instead.
Instead enqueue it properly with a full absolute URL that points to your theme, instead of using a relative URL in a hardcoded link tag.