Multiple Logstash instances as Kafka inputs

I am having trouble in making multiple Logstash instances read from a single Kafka topic.
The Logstash instances are all on the same server, together with Kafka and Zookeeper. When I check the topic with kafka-console-consumer and kafka-console-producer scripts everything is working fine and the messages are delivered.

I have a Filebeat instance that writes to a Kafka topic, the topic has 3 partitions and a replication factor 1 (single Kafka node). The Logstash instances have each their own config file with with distinct values for:

And a config file that looks like (the numbers for logstashN are set for each instance config file):

input {
    kafka {
      bootstrap_servers => "localhost:9092"
      topics => "netflow"
      group_id => "logstash"
      consumer_threads => 3
      client_id => "logstash1"
      client_rack => "rack-1"
      #partition_assignment_strategy => "round_robin"
filter {

    mutate {
            add_field => {"logstash_id" => "logstash1"}
output {
    elasticsearch {
      hosts => [""]
      index => "kafkagenerated-%{+YYYY.MM.dd}"
      id => "logstash1"

I start the Logstash instances each with their own config files like so:

/usr/share/logstash/bin/logstash -f /etc/logstash/conf.d/kafka-input.conf
/usr/share/logstash1/bin/logstash -f /etc/logstash1/conf.d/kafka-input.conf
/usr/share/logstash2/bin/logstash -f /etc/logstash2/conf.d/kafka-input.conf

They all start without error and consume the messages from the "netflow" topic. Also, when I check the group with:

bin/ --describe --bootstrap-server localhost:9092 --group logstash

I get an output that looks as if the consumers were load-balanced and that every Logstash instance did some work and indexed to Elasticsearch, this is run during high load when there is a lot of netflow data incoming (so the lag is understandable)

GROUP           TOPIC           PARTITION  CURRENT-OFFSET  LOG-END-OFFSET  LAG             CONSUMER-ID                                      HOST            CLIENT-ID
logstash        netflow         0          3189623         3245768         56145           logstash0-0-ff1448c5- 2742-4247-82fc-212d0a894f63 /      logstash0-0
logstash        netflow         2          1398036         1441479         43443           logstash0-2-9598982e-8565-4fd8-8f9a-85ada0b10f07 /      logstash0-2
logstash        netflow         1          1409207         1441478         32271           logstash0-1-7f14eb14-38d3-4f9f-bc25-0fdc582ce835 /      logstash0-1

HOWEVER when I check inside Elasticsearch all the documents have the same logstash_id, meaning that all the events are written by a single Logstash instance (I have checked the config files, every instance has its own number in the logstash_id).
To validate this I tried to shut down all instances but one and the indexing rate on the Elasticsearch is the same as when I have all three instances running.
My goal is to have as many Logstash instances as needed to get around 50k index/sec. In a production environment this would be on multiple machines .

Can someone please give me an idea how to troubleshoot this and how to get all the Logstash instances to work? Thank you in advance!

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