Ok, these code in iRedMail script:

perl -pi -e 's#(.*managesieve_default.*=).*#${1} "";#' config.inc.php

will set non default from dist file to /opt/www/roundcubemail/plugins/managesieve/config.inc.php:

$config['managesieve_default'] = "";

but these too:

$config['managesieve_default_headers'] = "";

We should keep:

$config['managesieve_default_headers'] = ['Subject', 'From', 'To'];

(roundcube added some code to avoid such values break the selector)