Empty aggregations, ES 6.7 nodes, RHLC upgraded from v6.3 to v6.8

[Hi_Jonk]

As part of a rolling upgrade, I am trying upgrade my high level client from v6.3 to v6.8 while talking to v6.7 nodes. Search is working fine, however, the aggregation queries we were using with the v6.3 client have stopped working after upgrading to the v6.8 client.

Example query:

SearchRequest {
        searchType = QUERY_THEN_FETCH, 
        indices = [indexName], 
        indicesOptions = IndicesOptions[ignore_unavailable = false, allow_no_indices = true, expand_wildcards_open = true, expand_wildcards_closed = false, allow_aliases_to_multiple_indices = true, forbid_closed_indices = true, ignore_aliases = false, ignore_throttled = true], 
        types = [], 
        routing = 'null', 
        preference = 'null', 
        requestCache = null, 
        scroll = null, 
        maxConcurrentShardRequests = 0, 
        batchedReduceSize = 512, 
        preFilterShardSize = 128, 
        allowPartialSearchResults = null, 
        localClusterAlias = null, 
        getOrCreateAbsoluteStartMillis = -1, 
        source = 
        {
            "query": {
                "range": {
                    "createdOn": {
                        "from": "1687338000000",
                        "to": "1687942807993",
                        "include_lower": true,
                        "include_upper": true,
                        "boost": 1.0
                    }
                }
            },
            "aggregations": {
                "composite_aggregation": {
                    "composite": {
                        "size": 2000,
                        "sources": [{
                            "aggregation_over_time": {
                                "date_histogram": {
                                    "field": "createdOn",
                                    "missing_bucket": false,
                                    "value_type": "date",
                                    "order": "asc",
                                    "interval": "1h"
                                }
                            }
                        }]
                    },
                    "aggregations": {
                        "childAggregation": {
                            "terms": {
                                "field": "field1.keyword",
                                "size": 10,
                                "min_doc_count": 1,
                                "shard_min_doc_count": 0,
                                "show_term_doc_count_error": false,
                                "order": [{
                                    "_count": "desc"
                                }, {
                                    "_key": "asc"
                                }]
                            },
                            "aggregations": {
                                "childAggregation": {
                                    "terms": {
                                        "field": "field2.keyword",
                                        "size": 10,
                                        "min_doc_count": 1,
                                        "shard_min_doc_count": 0,
                                        "show_term_doc_count_error": false,
                                        "order": [{
                                            "_count": "desc"
                                        }, {
                                            "_key": "asc"
                                        }]
                                    }
                                }
                            }
                        }
                    }
                }
            }
        }

We're using nested aggregations since multi terms weren't available for 6.7 nodes.

I'm assuming there are some parameters that were introduced in the 6.8 client that we aren't setting correctly. Any help would be appreciated.