Failed shards in search response, but no reasons

When running load tests with many concurrent threads, I'll occasionally see
a result like this:

{"took":1,"timed_out":false,"_shards":{"total":2,"successful":1,"failed":1},"hits":{"total":0,"max_score":null,"hits":[]}}

Notice the missing "failures" array. I also see nothing in the logs. Does
anyone have any idea why this would occur, and why it happens more
frequently when there are more concurrent searches?

I'm running v0.19.2

Thanks,
Josh

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