I am not able to reproduce on a 6.4.6 instance. Check if you are using dispatcher and then try to bypass dispatcher to test.Lastly, log a daycare ticket if this is still an issue.
I would recommend setting up a DEBUG logger on "org.apache.sling.i18n" and check whats getting logged when you add a new entry to the dictionary.Also, check if all indexes are valid from jmx console.