Exchange Online MailUser ExternalEMailAddress Attribute changes (Hybrid)

Hi Everyone,

i'm looking for a Error in Exchange Online, where my Teams Users have (partly) no Calendar, when their Mailbox is onPremise.

When i tried Autodiscover v2 for effected Users i got a redirect to a onmicrosoft.com Addressspace and not do my onpremise Address Space. Microsoft Told me that the ExternalEMailAddress Attribute at the Mailserver in ExchangeOnline is responsible for that. 

After investigation i saw that some MailUsers in EXO have a different ExternalEmailaddress Attribut that the PrimarySMTPAddress.

While the Investigation with the Premier Support they asked me to create new logs, I tried to enable an account my test accounts and saw that simply enabling the account in my onPrem AD (no further changes) and syncing to AzureAD changed the Attribute ExternalEmailaddress from the primarysmtpaddress to the remote Routing Address from the EmailUser. 

For me it seems that there is an error somewhere in the Sync-Managers in Azure but Microsoft keeps asking me for logs from my onprem Mailserver and running HCW again and stuff like that.

Set-Mailuser -externalemailaddress is of course not working, because it is a synced identity. 

Has everyone seen a same behavoir?

Cheers,

Sebastian

PS: Some Accounts turned themself ok after a while but not all..