Problem with Completion suggester

[dojutsu-user (Vaibhav Gupta)]

(Using ES 6.7)
I have an index and want to support search-as-you-type feature. For that, I want to try completion suggester but I'm having trouble in reindexing to change the mappings old index.

Here's the old index mappings

{
    "old-index": {
        "mappings": {
            "doc": {
                "properties": {
                    "content": {
                        "type": "text"
                    },
                    "project": {
                        "type": "keyword"
                    },
                    "title": {
                        "type": "text"
                    },
                    "version": {
                        "type": "keyword"
                    }
                }
            }
        }
    }
}

Here's the new test index mappings

PUT test-completion
{
    "mappings": {
        "doc": {
            "properties": {
                "content": {
                    "type": "text",
                    "fields": {
                        "autocomplete": {
                            "type": "completion",
                            "contexts": [
                                {
                                    "name": "project",
                                    "type": "category",
                                    "path": "project"
                                },
                                {
                                    "name": "version",
                                    "type": "category",
                                    "path": "version"
                                }
                            ]
                        }
                    }
                },
                "title": {
                    "type": "text"
                },
                "project": {
                    "type": "keyword"
                },
                "version": {
                    "type": "keyword"
                }
            }
        }
    }
}

Here's the reindexing query

POST _reindex
{
    "source": {
        "index": "old-index"
    },
    "dest": {
        "index": "test-completion"
    }
}

And here's the query which returns no results

POST test-completion/_search
{
    "suggest": {
        "autocompletion_suggest": {
            "prefix": "part of documentation",
            "completion": {
                "field": "content.autocomplete",
                "fuzzy": {
                    "fuzziness": "AUTO"
                },
                "contexts": {
                    "project": "xyz-project",
                    "version": "abc-version"
                }
            }
        }
    }
}

If the prefix is set to a or b, it returns results outside of context.
Where I'm doing wrong?