Currently, due to a Microsoft API limitation, there are some constraints with backup and restore of the following types of attachments when you protect Microsoft Exchange Online data:
This is an Exchange Online item such as, email or event or contact, etc., that is attached to another Exchange Online item as a .msg file.
Both backup and restore of the item attachment are not supported.
Recommendation: A workaround till the limitation is resolved is to use the Outlook Web App and attach the downloaded .msg files to emails and send them to the concerned parties. These emails and attachments will be backed up and can be restored.
This is a link to a folder or file such as a text file or a Word document on a OneDrive for Business cloud drive, or other supported storage locations, attached to an event, email.
Backup and Download of such attachments are supported today whereas Restore is not supported.
When will this limitation be resolved
This limitation will be resolved by April 2022.