Elasticsearch 5.3: Indexing child document winds up on different shard than parent

[BarryS]

Elasticsearch 5.3 indexing issue: Children sometimes wind up on different shards than their parent after indexing. This prevents the use of hasChild and hasParent queries against the index.

The index (myindex_v001) contains three types: tasks, documents, assignments. When creating the index, documents are mapped as children of tasks, and assignments are mapped as children of documents.

{
  "myindex_v001": {
    "mappings": {
      "tasks": {
        "_all": {
          "analyzer": "default",
          "search_analyzer": "default_search"
        },
        ...
}

{
  "myindex_v001": {
    "mappings": {
      "documents": {
        "_all": {
           "analyzer": "default",
          "search_analyzer": "default_search"
        },
        "_parent": {
          "type": "tasks"
        },
        "_routing": {
          "required": true
        },
        ...
}

{
  "myindex_v001": {
     "mappings": {
       "assignments": {
         "_all": {
             "analyzer": "default",
             "search_analyzer": "default_search"
         },
         "_parent": {
             "type": "documents"
         },
         "_routing": {
             "required": true
         },
         ...
}

All indexing is performed within a Java service. Java makes a REST call (PUT) to the url specifying the index, type, and parent_id. The same code is used to perform indexing on tasks, documents, and assignments.

When indexing documents (http://eshost:port/myindex/documents/{document_id}?parent={task_id}) each indexed document winds up on the same shard as its parent task as expected. hasChild queries work just fine for tasks/documents.

However, when indexing assignments (http://eshost:port/myindex/assignments/{assignment_id}?parent={document_id}) the assignment usually winds up being indexed on a shard that is different from its parent document. This is causing problems because a hasChild and hasParent query does not work with documents/assignments.

Can anyone suggest what may be going wrong with indexing the assignments (not being placed on same shard as documents)?