AD connect is always one way sync for objects (users, groups, contacts and devices). AD connect syncs objects from on-prem to Azure AD.
AD connect doesn't writeback user from Azure AD to on-premises AD. The user writeback preview feature was removed in the August 2015 update to Azure AD Connect.
This is also documented in our public document https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-preview#user-writeback
Currently we do not have this feature. However, this might be one of our upcoming features in AD connect.
You can submit the feedback regarding this in our Azure feedback portal.
This feedback is directly monitored by out product managers and they will be able to share any update on this if they have.
Let me know if you have any further questions on this.
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