Cin7 Core's API does not provide enough information about the shipping location in their API for Shiptheory to create a complete sender address. Therefore when you ship with Cin7 Core in Shiptheory your Shiptheory shipping location will be used as the sender address for all of your Shipments. This behaviour can be customised using rule overrides.
If you encounter any issues, please feel free to contact our support team. You can call us on 0117 403 4313 (UK) / +1 (629) 666-6726 (US) or reach us online here.