Invalid SoftMatch

We are syncing a security group. We have 15 members in this group. We have the synchronization errors for 2 users with the message "Invalid SoftMatch"

Unable to update this object because the following attributes associated with this object have values that may already be associated with another object in your local directory services: [Mail User01@keyman .com;]. Correct or remove the duplicate values in your local directory. Please refer to http://support.microsoft.com/kb/2647098 for more information on identifying objects with duplicate attribute values.

Tracking Id: 5a08b1b5-0b1c-4794-bafc-58ce22b35734
ExtraErrorDetails:
[{"Key":"ObjectIdInConflict","Value":["a8bad58c-4a17-490c-82d1-5b45a200bdff"]},{"Key":"AttributeConflictName","Value":["Mail"]},{"Key":"AttributeConflictValues","Value":["User01@keyman .com"]}]

We have done everything we could find googling to correct this and we are unable to.

We have checked the immutableID of these 2 users on Azure AD and they do not have an immutableID. However all the other 13 users have an immutableID.

We deleted these users from Azure AD and ran the sync again, and everything synced perfect. However, the user lost all items in his Mailbox & Onedrive, it was like he had a new fresh mailbox and onedrive.

1. How can we set an immutableID for these users without having to delete them from Azure AD or M365 Active Users