As you are may be aware, Microsoft has released an updated version of Azure AD Connect to support the employeeLeaveDateTime attribute (see https://t.co/dzZs8LWyHb).
Well, there has been a bug where the new employeeLeaveDateTime attribute was not syncing correctly.
If you have been already implemented it using the AAD Connect version 2.1.19, you need to upgrade it (even if you are not using it, it is important to run the latest version) and then apply the below.
If you have been already implementing the new employeeLeaveDateTime attribute in AAD Connect, you will need to update the rule you have implemented to ensure the correct attribute is being used.
Then you will need to remove any objects in the AAD connector space using the Remove-ADSyncCSObject PowerShell command and run a full sync.
You can download the new version – 2.1.10 – here https://www.microsoft.com/en-us/download/details.aspx?id=47594