css style cannot correctly livereload in RStudio or browser when blogdown::serve_site()

Why does my custom CSS (custom.css) work correctly on my personal website after deploying to Netlify but fail to apply when serving the site locally with blogdown::serve_site() in RStudio or browser? Initially, the styles load, but after navigating pages, they reset. The CSS is correctly linked in baseof.html as follows:

<link rel="stylesheet" href="https://xxx/libs/bootstrap/5.2.0/css/bootstrap.min.css" integrity="sha512-XWTT" crossorigin="anonymous" />
<link rel="stylesheet" href="{{ "custom.css" | relURL }}">

The file is placed in the static/ folder, Hugo is updated to the lastest. The issue occurs in both RStudio 2023.06 and 2024.12 .

What could be causing the CSS to fail in live reload?"

Last but not least, this matter remains after I deleted "bootstrap.min.css".

I try to move custom.css in the static/ dir to css/custom.css,it works. But why ?

<link rel="stylesheet" href="{{ "css/custom.css" | relURL }}">