The consumer does not consume all partitions fairly

**Environment Information**
- OS [CentOS Linux]:
- Node Version [12 and 14]:
- NPM Version [6.14.5]:
- node-rdkafka version [2.9.0]:

**node-rdkafka Configuration Settings**
I use non-following mode consumer and each batch contains 2000 records, consumer configuration:

```
{
groupId: 'frg',
bootstrapServer: '10.120.1.20:9092',
topic: 'rdo-stocks-v1',
clientId: 'rdo-stock-quantity',
enableAutoCommit: true,
enableAutoOffsetStore: true,
socketKeepaliveEnable: 'true',
securityProtocol: 'SASL_PLAINTEXT',
saslMechanisms: 'SCRAM-SHA-512',
saslUsername: 'rdUser',
saslPassword: 'secret',
consumerMaxPollInterval: '200000',
consumerAutoOffsetReset: 'earliest',
consumerCommitInterval: 500,
consumerPartitionAssignmentStrategy: 'roundrobin',
debug: 'fetch',
consumerHeartbeatInternalMs: 10000,
consumerSessionTimeout: 20000,
consumerFetchMessageSizeByte: 1048576,
messageMaxBytes: 1548576,
consumerQueuedMaxMessagesKbytes: 1048576,
consumerMaxPollRecords: 2000,
consumerDefaultTimeout: 100
}
```
UV_THREADPOOL_SIZE is set to 30.
The consumer run on in a docker container and each consumer runs on a different host (8 cores).

**Additional context**

The consumer consumes messages well, but as we can see in the picture below, each Kafka broker is the leader of two partitions and the consumer consumes only 1 partition:


Initially, the consumer commit messages manually, I changed it to auto-commit and i have the same result. #180
**Note**
We have a Kafka cluster with 4 brokers, and each one has only 2 cores(to this consumer we add a sink and source connector), is that enough?

Any help is welcome

Thank's
Samir