ES msearch responses 404 when using GET while it reponses 200 when using POST

Could you help explain why the behavior is different between GET and POST? And advise how to make it work when using GET? Thanks in advance.

curl --location --request GET 'http://kibana.xxx.xxx.xx:5601/elasticsearch/_msearch'
--header 'Content-Type: application/x-ndjson'
--header 'kbn-version: 6.8.3'
--data-raw '{"index": "-xxx-"}
{"aggs":{"2":{"terms":{"field":"env_name","size":50,"order":{"_key":"asc"}}}},"size":0,"_source":{"excludes":},"stored_fields":["*"],"script_fields":{},"docvalue_fields":[{"field":"@timestamp","format":"date_time"},{"field":"timestamp","format":"date_time"}],"query":{"bool":{"must":[{"range":{"@timestamp":{"gte":1577669673076,"lte":1578274473076,"format":"epoch_millis"}}}],"filter":[{"match_all":{}},{"bool":{"filter":[{"bool":{"should":[{"bool":{"should":[{"match":{"env_name":"xxx"}}],"minimum_should_match":1}},{"bool":{"should":[{"match":{"location":"xxx"}}],"minimum_should_match":1}}],"minimum_should_match":1}},{"bool":{"should":[{"match_phrase":{"uri":"/xxx/xxx/xxx"}}],"minimum_should_match":1}}]}}],"should":,"must_not":}},"timeout":"30000ms"}

{
"statusCode": 404,
"error": "Not Found",
"message": "Not Found"
}

curl --location --request POST 'http://kibana.xxx.xxx.xx:5601/elasticsearch/_msearch'
--header 'Content-Type: application/x-ndjson'
--header 'kbn-version: 6.8.3'
--data-raw '{"index": "-xxx-"}
{"aggs":{"2":{"terms":{"field":"env_name","size":50,"order":{"_key":"asc"}}}},"size":0,"_source":{"excludes":},"stored_fields":["*"],"script_fields":{},"docvalue_fields":[{"field":"@timestamp","format":"date_time"},{"field":"timestamp","format":"date_time"}],"query":{"bool":{"must":[{"range":{"@timestamp":{"gte":1577669673076,"lte":1578274473076,"format":"epoch_millis"}}}],"filter":[{"match_all":{}},{"bool":{"filter":[{"bool":{"should":[{"bool":{"should":[{"match":{"env_name":"xxx"}}],"minimum_should_match":1}},{"bool":{"should":[{"match":{"location":"xxx"}}],"minimum_should_match":1}}],"minimum_should_match":1}},{"bool":{"should":[{"match_phrase":{"uri":"/xxx/xxx/xxx"}}],"minimum_should_match":1}}]}}],"should":,"must_not":}},"timeout":"30000ms"}

{
"responses": [
{
"took": 14,
"timed_out": false,
"_shards": {
"total": 7,
"successful": 7,
"skipped": 0,
"failed": 0
},
"hits": {
"total": 525111,
"max_score": 0,
"hits":
},
"aggregations": {
"2": {
"doc_count_error_upper_bound": 0,
"sum_other_doc_count": 0,
"buckets": [
{
"key": "xxx",
"doc_count": 682
},
...
]
}
},
"status": 200
}
]
}

The request should request to ES rather than Kibana, it works after changing to request http://kibana.xxx.xxx.xx:9200/_msearch.