@import and @use fail when using external css

[danbuntu]

Describe the bug

I have an external SCSS linked set in theme_boost_union | extscssurlpost that is working fine until I add @import or @use to the file. I've tried with relative path, full path and url. I have used this method in a child theme based of boost.

I get the following error:

Debugging: Error while compiling SCSS: parse error: failed at @use "/mnt/web/moodle4test/skillset/scss/moodle4test/staging/test.scss";\r (stdin) on line 38, at column 1 in \n* line 1532 of /lib/outputlib.php: call to debugging()\n* line 1270 of /lib/outputlib.php: call to theme_config->get_css_content_from_scss()\n* line 69 of /theme/styles_debug.php: call to theme_config->get_css_content_debug()\n, referer: https://moodle4test.skillset.co.uk/admin/tool/task/schedule_task.php

I'm keen to use @use if possible as it would let me share partials across a number of different stylesheets/ moodles

To Reproduce
Steps to reproduce the behavior:

1. Link an external styles sheet
2. Check it works
3. Add an @use or @import
4. Run the purge boost union purge cache task and look the error logs

Expected behavior
Be able to use @use and @import in order to include other style sheets and partials

Additional context
Moodle: 4.1.10
Boost_union: 2023010568