Getting records rejected during bulk insert, which are not related to the '400 Failed to parse' reason

Using Bulk API to upsert mini-batches of records to Elasticsearch 5.1

When I try to upsert a badly formatted record (hence getting a 400 Failed to parse), most times ES rejects just that record in that batch. But sometimes, ES rejects that record as well as other records in that batch that do not have that issue (confirmed by successfully upserting those records in a different batch).

Is this expected?
How to understand this inconsistent behavior?

Huh, this is interesting. I seem to recall there being a slightly similar issue logged about Bulk endpoint in the Elasticsearch project. Do you have any example records you could share that would reproduce the error?