Can’t get Logstash to read existing Kafka topic from start

I'm trying to consume a Kafka topic using Logstash, for indexing by Elasticsearch. The Kafka events are JSON documents.

We recently upgraded our Elastic Stack to 5.1.2.

I believe that I was able to consume the topic OK in 5.0, using the same settings, but that was a while ago so perhaps I'm doing something wrong now, but can't see it. This is my config (slightly sanitized):

input {
    kafka {
        bootstrap_servers => "host1:9092,host2:9092,host3:9092"
        client_id => "logstash-elastic-5-c5"
        group_id => "logstash-elastic-5-g5"
        topics => "trp_v1"
        auto_offset_reset => "earliest"
    }
}

filter {

    json {
        source => "message"
    }

    mutate {

        rename => { "@timestamp" => "indexedDatetime" }

        remove_field => [
            "@timestamp",
            "@version",
            "message"
        ]
    }
}

output {
    stdout { codec => rubydebug }

    elasticsearch {
        hosts => ["host10:9200", "host11:9200", "host12:9200", "host13:9200"]
        action => "index"
        index => "trp-i"
        document_type => "event"
    }
}

When I run this, no messages are consumed, no sign of activity appears in the log after "[org.apache.kafka.clients.consumer.internals.ConsumerCoordinator] Setting newly assigned partitions", and in Kafka Manager the consumer appears to immediately appear with "total lag = 0" for the topic.

This version of the Kafka plugin stores consumer offsets in Kafka itself, so each time I try to run Logstash against the same topic, I increment the group_id so in theory, it should start from the earliest offset for the topic.

Any advice?

EDIT: It appears that despite setting auto_offset_reset to "earliest", it isn't working - it's as if it's being set to "latest". I left Logstash running, then had more entries loaded into the Kafka queue, and they were processed by Logstash.