A cloud-based identity and access management service for securing user authentication and resource access
Hello @Markus Schädlich
I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this.
Issue: email-enabled security group synchronized from the local active directory is not deleted in Exchange online after entra id sync
Azure Trusted signing failure
Solution: The Group had an automatic Group license assignment. The Group was deleted After removing the assignment.
Issue is resolved by @Markus Schädlich
If I missed anything please let me know and I'd be happy to add it to my answer, or feel free to comment below with any additional information.