Secondary Google Workspace domains not showing up in Business Manager

There is no community for Apple Business Manager so I'm posting here.

In Google Workspace I have a primary and multiple secondary domains. In ABM I verified the important domains and configured federated access.

Unfortunately only Users from the primary domain are being synced into ABM.

ABM should be able to handle multiple domains from federated access as far as I read online?!

Hope anybody can help.

Thanks!

Nicolai