There is nothing wrong with CSV or text base formats. You can even use AWS Athena to query CSV feels stored in S3. It's a good format for data import/export, that many systems can natively understand or have tools to parse, given it's known how to interpret data.

It was a data pipeline issue. Software has little to do with it. If they received data in json and tried to interpret it as CSV, the same could have happened. I believe Excel even warns when you open file that has too many rows.