Strange problem with LDAP username length

Hey Guys
I’ve got a strange problem with our companies nextcloud instance. We use the LDAP app for user authentication and it works just fine.

But today a user called me and said that he can’t log in with his AD credentials.
The result of my research was that nextcloud imported the user, but limited the username to 20 characters. And he’s not the only one, this is for all users with a login with more than 20 characters. In our LDAP (AD) the username is complete. I didn’t found any parameter to adjust the max length of the usernames.

Nextcloud version: 15.0.5
LDAP user and group backend version: 1.5.0
LDAP: Active Directory (Windows Server 2012 R2)

I hope you can help me

Greets from germany

EDIT:
Problem persists in Nextcloud Version 16.0.7 with LDAP App Version 1.6.0. Any help would be appreciated.