Custom SCSS doesn't work

The theme_enlightlite | customcss setting (Site administration → Appearance → Themes → ENLIGHTLITE THEME SETTINGS → MISCELLANEOUS ITEMS, Custom SCSS) doesn’t appear to have any effect. Any CSS added here appears to be ignored.

This issue was observed in theme version 2019120701 and Moodle 3.8. However, with theme version 2020073000 and Moodle 3.9 the problem does not occur.