Hybrid Resource Forest / Mail User with Linked Mailbox

Hello all,

I have the problem that no mail users are created under contacts in Exchange Online. I have read through these two guides:

https://jaapwesselius.com/2018/04/26/exchange-resource-forest-and-office-365-part-ii/

https://www.msxfaq.de/cloud/exchangeonline/hybrid_ressourceforest.htm

Both guids explain the same thing. The only problem I have is that no mail user is created when a linked user is synchronized to the cloud.

The two users are made Linked Mailboxes by the command "Set-User -LinkedMaster...". When the user is synchronized without a link, it shows up in Exchange Online under Contacts and I can start a migration batch with this user.

The user also exists only once in the cloud, as it should, since it is linked by ObjectSID and msExchMasterAccountSid.

So I can't create a migration batch in which the user should be migrated. Unfortunately the migration batch can't find the user...

Did I make a mistake when setting up the Azure AD Connect or what could be the reason?

Thanks
Niklas