I recently opened a case with both Fivetran and SAP Concur regarding missing Attendee records. The response from Concur states there is an issue with the V3 Attendee API(s) and that we should start using the V4 APis instead. Below is Concur's response:

Hello,

the Attendees v3 indeed have problems. We strongly recommend the usage of Attendees v4, which is very similar, but more consistency. This is the recommendation from our R&D team.

In order to test I made a similar code in Python, exporting the UUIDs to a CSV files, using the Attendees v4 API, and all went well and was consistent with the results.

Kind Regards,
Gabriel
Integration Consultant - Client Web Services
SAP Concur Managed Data