Then Thunderbird doesn't use SORT for sorting. Roundcube trusts in server responses. To re-sort messages we would need to fetch Date/INTERNAL date value for all messages in a folder which wouldn't be good for performance. This is exactly what we do when SORT is not supported by the server.
OK, but in this case there should be a config option not to use SORT even if server reports CAPABILITY SORT.
List info: http://lists.roundcube.net/dev/ BT/aba52c80