Timeout issue when using reindex API

Hello. I am using elasticsearch and kibana with version 7.5.0 now.
My cluster consists of 1 master node, 2 data nodes and 1 coordi node.
I tried to reindex an index with about 200k documents to a new index within the cluster using _reindex API.
However, all documents are not indexed properly. When number of docs reindexed reaches request's batch size, reindexing suddenly stops and nothing happens.

POST _reindex?wait_for_completion=false
{
  "source": {
    "index": "old_index",
    "size": 10000
  },
  "dest": {
    "index": "new_index"
  },
  "script": {
    "source": 
    """
      ctx._source.new_field1 = ctx._source.remove("old_field1");
      ctx._source.new_field2 = ctx._source.remove("old_field2")
    """
  }
}

Above is the request I sent. In this case, only 10k docs are reindexed.

{
  "index": "new_index",
  "type": "_doc",
  "id": {{docs_id}},
  "cause": {
    "type": "mapper_exception",
    "reason": "timed out while waiting for a dynamic mapping update"
  },
  "status": 500
}

Above is the failure object of the task(reindexing).

I googled about it few hours but I can not figure out why.
Is there something I missed?

Thanks in advance.