When creating a call from an Events Management record, the Attendees of the event are not pre-selected as Call Attendees.
If the Enable Child Account Veeva Setting is set to true, the user needs visibility to a valid Child Account record. In addition visibility to both account Ids from the Child Account and Parent Account fields is needed to have suggested event attendees show.
Grant the user proper access to the Child Account record and to the Parent Account record.
CRM Help Documentation: Events Management