json-mapping | existing JSON object with this mapping module | JSON Processing library

As far as I can see - at least one issue is with your choice of format - you're using json instead of multijson, and you should switch to the latter.

The fact that it works with your .ingest inline command is due to the payload there being different, and complying with the json format (a single record in a single line)

From your code: Format = DataSourceFormat.json,

From the Documentation:

The JSON format

Azure Data Explorer supports two JSON file formats:

json: Line separated JSON. Each line in the input data has exactly one JSON record.

multijson: Multi-lined JSON. The parser ignores the line separators and reads a record from the previous position to the end of a valid JSON. For more information, see JSON Lines.