SCSS file not found - cannot add/customize/override scss for a single site

# Bug Report
Adding this typoscipt:

```typescript
page.includeCSS {
theme-999 = fileadmin/template-site/common.scss
}
```

### Expected behavior

This is a fast and simple way to add custom SCSS files processed with the bootstrap SCSS files

### Actual behavior

TYPO3 v. 16.0.0 with boostrap package 16.0.0 return this error

```typescript
`/var/www/example.com/web/FAL:1:/template-site/common.scss` file not found for @import: /var/www/example.com/web/FAL:1:/template-site/common.scss on line 1, at column 0
```

### Notes

- running this code under TYPO3 v.12-13 and previous boostrap, all is ok and fine
- as you can see above, instead to use the folder name **'fileadmin'** the compiler resolved with **FAL:1:**
- obviously the files common.scss exist and the document is accessible
- if you change the line: **theme-999 = fileadmin/template-site/common.scss** with **theme-999 = FILE:fileadmin/template-site/common.scss** no errors are displayed but the file is not considered anyway

## Versions

TYPO3 Version 14.0.2
Classic mode
webserver Apache
PHP 8.2.30
MySQL 10.11.14-MariaDB-0+deb12u2
Production
o.s. Linux 6.1.0-42-amd64
bootstrap packge 16.0.0

Reproduce the reported TypoScript includeCSS configuration with fileadmin/template-site/common.scss in the Bootstrap Package environment described, then trace how the SCSS compiler resolves that path and the FILE: variant. Done means a site-specific SCSS file is found, processed with the Bootstrap SCSS files, and included in the generated styling without the FAL:1: path error.

Written by the indexing model from the issue text.