Sharepoint - csom FieldUserValue.LookupValue return users displayname, not loginname

The FieldUserValue.LookupValue depends on how the field is configured in the list. It may return the display name, email address, login name. This can be updated at any time by someone with design permissions on the list.

You can use the FieldUserValue.Email property to call the Utility.ResolvePrincipal method to get the LoginName. See https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/get-user-identity-and-properties-in-sharepoint#retrieve-current-website-user-identity-by-using-the-web-object for details.