Querying Exchange LDAP servers

I'm having a discussion with a user about why Thunderbird and their LDAP 
connection doesn't return results for autocompletion [1].

At the moment this seems to boil down to the search parameters:

(sn=*sur*) will match
(sn=sur*) doesn't match.

Has anyone seen this before, is it a known bug in the server or something?

Thanks
Mark.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=668087