Index may cause issue during CSV import. It happens when there is project with single issue type only and there is required custom field with Context set to that project. During manual creation is everything OK.

Replication

1. Create new JIRA project, with exactly one issue type.
2. Add new custom field. And restrict context for that issue type.
3. In project Issue Field Configuration scheme set all fields to optional, except Sumary (It is mandatory to have it mandatory) and the new custom field.
4. Create issue normally - it works
5. Create issue by CSV import - Import fails because that field is always treated as null.

Note: Because there is only single issue type, import wizard do not offer Issue Type for mapping. My theory is that importer checks context before it actually applies issue type - automatically.

Workaround: Set context into project instead of Issue type.