Wordpress plugin not finding Composer Autoloader

When I try to use Autoload in my Wordpress plugin I get an unexpected error:

not finding

Warning: require(/home/251650.cloudwaysapps.com/geytjjwugd/public_html/wp-content/plugins/gik25-microdata/vendor/composer/../phpunit/phpunit/src/Framework/Assert/Functions.php): failed to open stream: No such file or directory in /home/251650.cloudwaysapps.com/geytjjwugd/public_html/wp-content/plugins/gik25-microdata/vendor/composer/autoload_real.php on line 55

Fatal error: require(): Failed opening required '/home/251650.cloudwaysapps.com/geytjjwugd/public_html/wp-content/plugins/gik25-microdata/vendor/composer/../phpunit/phpunit/src/Framework/Assert/Functions.php' (include_path='.:/usr/share/php') in /home/251650.cloudwaysapps.com/geytjjwugd/public_html/wp-content/plugins/gik25-microdata/vendor/composer/autoload_real.php on line 55

• I've uploaded the files via SFTP
• Called require_once(vendor/autoload.php); in the plugin entry file (it works) *The autoload.php file contains require_once DIR . '/composer/autoload_real.php';