Currently adjusting offsets.topic.num.partitions after the offset topic is created is not supported. Meaning that the number of partitions will not change once the topic has been created.
However, changing the value in the configuration should not cause issues and instead simply be ignored. Currently this is not the case.
When the value of offsets.topic.num.partitions is changed after the offset topic is created the consumer group partition assignment completely changes even though the number of partitions does not change.
This is because kafka.server.OffsetManager.partitionFor(group: String) uses the configured value and not the value of the actual topic.