So phpbb does not check for updates to the LDAP mail attribute: you need to change the email in phpbb instead. phpbb.LDAP e-mail attribute: mail
Set this to the name of your user entry e-mail attribute (if one exists) in order to automatically set the e-mail address for new users. Leaving this empty results in empty e-mail address for users who log in for the first time.
I think this is reasonable behavior, because it allows defaulting to the email address in LDAP, but does not prevent customizing the email address per-service. In fact, this is the same behavior as we have in Trac now.
Samuli