Routing to an Index Partition - Custom Routing

I have configured the index such that custom routing values will go to a subset of the shards rather than a single shard by providing the index level setting "index.routing_partition_size" at index creation.

Index Creation:

PUT /test_sharding.partition
"settings": {
"number_of_shards": 3,
"routing_partition_size": 2
"mappings": {
"_routing": {
"required": true
PUT /test_sharding.partition/_doc/1?routing=202109
"data" : "test data"

With the same routing key i have inserted around 300k documents in to the index. I expect that docs should distribute between 2 shards, because "routing_partition_size" is configured as 2. But all the docs are going to only one shard.

I'm not getting much information on this. Can you please help on this.