Class 'League\Csv\Writer' not found

Problem/Motivation

Error: Class 'League\Csv\Writer' not found in Drupal\csv_serialization\Encoder\CsvEncoder->encode() (line 150 of /public_html/modules/csv_serialization/src/Encoder/CsvEncoder.php)

Steps to reproduce

Attempting to export a view.

Current setup

1. Drupal: 8.9.20 (Update underway but not completed yet)
2. PHP: 7.3.33
3. CSV Serialization: 8.x-2.0 (Installed via Composer)
4. League/CSV: 9.6.2 (Installed via Composer, confirmed via json and lock file)

I've confirmed all the files have the correct user:group and permissions.

Caches have been cleared, modules and libraries have been re-installed (via composer), other newer versions (9.1 and ^9) have been tried.
Same error constantly.