Contacts getting stuck on the OIC Connector

We are using the Eloqua OIC Connector to pass contacts into an OIC integration from a program canvas. The response back to Eloqua includes all the contacts that were received in the OIC request.

The contacts remain stuck in the Eloqua Canvas step with a status of 'Being processed by app' even though they are 'succeeded' in the integration:

The format of the data we are passing back into the response to Eloqua is:

The fields being passed into the request are different to the fields in the response, not sure if that would make any difference? Would null values cause any issue? We're a bit stumped as to why this is failing so any help would be appreciated!