If the configured SXP Devices exist in the ISE instance from which you're taking the backup and are not created in the ISE instance to which you are restoring the configuration from that backup, you might want to open a TAC case to investigate. That seems like a very important configuration missing from the restore.
The only method to export/import the SXP Devices would be using the APIs. You would need to use GET calls to retrieve the settings for each individual connection on the source instance, then use either individual POST calls or a bulk PUT call on the target instance.
https://developer.cisco.com/docs/identity-services-engine/latest/#!sxpconnections
