Note:
None of the Azure AD Connect v2.x releases are released for automatic upgrade. Manual upgrades are required to gain the new functionality and security levels once you’re on the Azure AD Connect v2 path.

What’s Fixed

Microsoft addressed a bug in version 2.0.88.0 where, under certain conditions, linked mailboxes of disabled users were getting deleted.

ABOUT LINKED MAILBOXES

A linked mailbox is a mailbox that’s associated with an external account.

The resource forest scenario is the prime example of a situation in which you would want to associate a mailbox with an external account. In a resource forest scenario, user objects in the Exchange forest have mailboxes, but the user objects are disabled for logon. You must associate these mailbox objects in the Exchange forest with enabled user objects in the external accounts forest(s).

While the resource forest scenario is one of the most obvious reasons for linked mailboxes, linked mailboxes can also be remnants of botched and/or incomplete Active Directory migrations using the Active Directory Migration Tool (ADMT) or any 3rd party migration solution(s).

A linked mailbox can also come to life when you orphan and then reattach an Exchange mailbox to another user, for instance a recreated user in case of an accidental deletion.

Version information

This is version 2.0.89.0 of Azure AD Connect.
This release in the 2.x branch for Azure AD Connect was made available for download as a 153 MB weighing AzureADConnect.msi on December 22, 2021.

You can download the latest version of Azure AD Connect here.

Read the full artcile here:https://dirteam.com/sander/2021/12/23/azure-ad-connect-v2-0-89-0-addresses-an-issue-with-disappearing-linked-mailboxes/