In case an archived item doesn't have PR_ICON_INDEX property, Enterprise Vault restores the item with PR_ICON_INDEX property set to "0". Enterprise Vault considers the value "0" as a common default value for numeric properties including PR_ICON_INDEX. But Outlook doesn't define value "0" for PR_ICON_INDEX property and treats it as Unread icon specification.
Note that actual default value for PR_ICON_INDEX defined by Outlook / Microsoft is "-1" (or 0xFFFFFFFF).
Enterprise Vault 11.0.0 Cumulative Hotfix 3 Release
https://www.veritas.com/docs/000024104
Enterprise Vault 11.0.1 Cumulative Hotfix 1 Release
https://www.veritas.com/docs/000024613
Enterprise Vault 12.0
Enterprise Vault for Microsoft Exchange, and RestoreToPSTOption in WebApp.ini is enabled.