User Sync error when new user registers on behalf of a business
When someone contributes on behalf of an organisation, the membership is applied to the organisation, not to the individual. By default, CiviCRM WordPress Member Sync (perhaps incorrectly) creates a user for the organisation and then applies the appropriate role based on the membership in question. You can override this behaviour (i.e. cancel creating the organisation user) by using the civi_wp_member_sync_auto_create_wp_user filter:

https://github.com/christianwach/civicrm-wp-member-sync/blob/master/civi-wp-ms-users.php#L603-L612