Import Set REST API - JSON array only processes first record - Is this by design or limitation?

[kakunurisat]

Hi Community,

We are building a high volume identity provisioning system that needs
to provision 100,000+ records to ServiceNow sys_user table.

We created an Import Set staging table (u_iam_user_import) with a
Transform Map to sys_user with Coalesce on user_name field.

PROBLEM:
When we POST a JSON array to the Import Set REST API, only the
FIRST record gets processed. All other records are ignored.

TEST:
POST /api/now/import/u_iam_user_import
Body:
[
{"u_user_name": "user001", "u_last_name": "Test001"},
{"u_user_name": "user002", "u_last_name": "Test002"},
{"u_user_name": "user003", "u_last_name": "Test003"}
]

RESULT:
- Only user001 created in sys_user
- user002 and user003 NOT created
- Response only returns 1 result

We also tried insertMultiple endpoint:
POST /api/now/import/u_iam_user_import/insertMultiple
Body: {"records": [...]}
Result: All records inserted into staging but all fields empty → skipped

QUESTIONS:
1. Is JSON array processing limited to first record by design?
2. Does this behave differently on production/enterprise instance vs PDI?
3. Is insertMultiple the correct bulk approach? Why are fields empty?
4. What is the RECOMMENDED approach to provision 100,000+ records
to sys_user via REST API in minimum time?
5. Is there a Batch API or any other approach for true bulk operations?

Instance: Personal Developer Instance (PDI)
Version: (check your version and add here)

Any help appreciated. Thank you!