Hey A.L.E.C., thanks for hitting back.
You mean the filed message_sort_col in roundcube's settings file?
In the config/defaults.inc.php file it says:
// default messages sort column. Use empty value for default server's sorting,
// or 'arrival', 'date', 'subject', 'from', 'to', 'fromto', 'size', 'cc'
$config['message_sort_col'] = '';
I've checked and in all my servers it is set to empty:
/var/www/config # grep -r message_sort_col .
./defaults.inc.php:$config['message_sort_col'] = '';
I tried changing it to 'None' as you suggested but it didn't take any effect. Roundcube is still issuing the SORT command before FETCH.