Question:

Why are custom fields beyond Custom20 not supported for Attendees by Veeva Concur integration?

Answer:

Veeva CRM integration with Concur uses version 1.1 of the Concur API, which only supports custom fields Custom1 to Custom20.  Fields beyond this are not supported by the implementation.

Recommend customers which are struggling with this limitation to submit an enhancement request to gain traction on development.

Related Documentation:

Concur Developer API Reference: 
Attendees v1.1: https://developer.concur.com/api-reference/expense/attendees/v1dot1.attendees.html