Partial synchronization of roles / groups

Hello,
First of all, thanks for this great module.

On a Durpal-6.22 and phpBB3.0.8 fresh install, I can't properly sync roles / groups.

Setup :

Master: Drupal
role1 matches group1
role2 matches group 2

procedure : adding and editing users in Drupal, checking results in phpBB

test sequence :

Drupal: create user1 without any assigned role
Drupal: create user2 with role2 assigned to it
phpBB: both users have been created but user2 was not added to group2
Drupal: edit user2 (change nothing, just save)
phpBB: now, user2 has been added to group2
Drupal: edit user1, assign role1, save
phpBB: user1 has been added to group1
Drupal: edit user2, unassign role2, save
phpBB: user2 still belongs to group2
Drupal: edit user2, assign role1, save
phpBB: user2 has been added to group1 (belongs to both groups but endorses only role 1)
Drupal: delete user2
phpBB: user2 is deleted and removed from it's groups

Summary :

When a user is created in Drupal, it's roles are not synced in phpBB
When a role is added in Drupal to an already synced user, the new role is properly synced
When a role taken away from an existing user in Drupal, the user still belongs to the corresponding group in phpBB

or acconding to the Create/Update/Delete scheme :

Create : user only, no groups
Update : add to group only, no retract from group
Delete : OK

I hope it can help those of you who have programming skills...