OpenLDAP non primary group members not syncing

I am using a OpenLDAP Directory profile to sync my users from my FreeIPA instance.
The User sync works great and without a Problem.
Syncing groups also works but group membership is not properly assigned.

The primary groups get properly assigned to each user.
I have a group called “pbx_users” wich indicates general access to the PBX.
Every user that is synced must be member of the group.
But if i check in FreePBX no user is assigned to it.

I’ve checked the wiki and multiple other Forum threads without a solution.
The OpenLDAP Group configuration also contains a Field that is not documented in the wiki called “Group Member Identifier Attribute”.
Here is a Screenshot of my Group configuration


Here is the Group sync part of the userman sync log (usernames replaced):

Updating All Groups
	ldapsearch -w ... -H "ldap://<our FreeIPA server>:389" -D "uid=freepbx,cn=sysaccounts,cn=etc,dc=<company name>,dc=de" -b "cn=groups,cn=accounts,dc=<company name>,dc=de" -s sub "(|(cn=pbx_*)(objectclass=mepmanagedentry))"
	Retrieving all groups...
	Got 12 groups
	Working on user1
		Updating user1
	Working on user2
		Updating user2
	Working on user3
		Updating user3
	Working on user4
		Updating user4
	Working on user5
		Updating user5
	Working on user6
		Updating user6
	Working on user7
		Updating user7
	Working on ttestington
		Updating ttestington
	Working on user8
		Updating user8
	Working on user9
		Updating user9
	Working on pbx_users
		Updating pbx_users
	Working on user10
		Updating user10
Finished adding users from non-primary groups
Updating Primary Groups
	User user1 primary group is user1
	User user2 primary group is user2
	User user4 primary group is user4
	User user5 primary group is user5
	User user6 primary group is user6
	User user7 primary group is user7
	User user8 primary group is user8
	User user9 primary group is user9
Executing User Manager Hooks
	Updating User user1...done
	Updating User user2...done
	Updating User user4...done
	Updating User user5...done
	Updating User user6...done
	Updating User user7...done
	Updating User user8...done
	Updating User user9...done
	Updating Group user1...done
	Updating Group user2...done
	Updating Group user3...done
	Updating Group user4...done
	Updating Group user5...done
	Updating Group user6...done
	Updating Group user7...done
	Updating Group ttestington...done
	Updating Group user8...done
	Updating Group user9...done
	Updating Group pbx_users...done
	Updating Group user10...done
Finished

Hopefully someone can help me getting the group assignment to work.