Regression in QueryString/Percolator behavior on unmapped fields between 7.6.2 and 7.7.0

[ruke47]

Elasticsearch version (bin/elasticsearch --version): 7.7.0 (problem persists through 7.9.2)

Plugins installed: none

JVM version (java -version):

openjdk version "11.0.6" 2020-01-14
OpenJDK Runtime Environment AdoptOpenJDK (build 11.0.6+10)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 11.0.6+10, mixed mode)

OS version (uname -a if on a Unix-like system):
reproduced on:
Darwin Kernel Version 19.6.0: Thu Jun 18 20:49:00 PDT 2020; root:xnu-6153.141.1~1/RELEASE_X86_64 x86_64
4.4.0-1026-aws #35-Ubuntu SMP Thu Jul 20 21:59:09 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

Description of the problem including expected versus actual behavior: In 7.6.2-, I am able to set map_unmapped_fields_as_text: true on an index, create a percolator_query that consists of a query_string query, and match documents that contains unmapped fields against it. This functionality appears to break in 7.7.0.

Steps to reproduce:
The below results in 1 hit in 7.6.2, but 0 hits in 7.7.0:

curl -XPUT "localhost:${PORT}/percolation-index" -H 'Content-Type:application/json' -d '
{
  "settings": {
    "percolator": {
      "map_unmapped_fields_as_text": true
    }
  },
  "mappings": {
    "properties": {
      "percolator_field": {
        "type": "percolator"
      }
    }
  }
}'
curl -XPOST "localhost:${PORT}/percolation-index/_doc?refresh=true" -H 'Content-Type:application/json' -d '
{
  "percolator_field": {
	"query_string": {
	  "query": "message:test"
	}
  }
}'
curl -XPOST "localhost:${PORT}/percolation-index/_search?pretty" -H 'Content-Type:application/json' -d '
{
  "query": {
    "percolate": {
      "field": "percolator_field",
      "document": {
        "message": "test stuff"
      }
    }
  }
}'

Provide logs (if relevant): n/a

(Crossposted from: https://discuss.elastic.co/t/some-change-in-percolation-querystringquery-mapping-between-7-6-2-and-7-7-0/250579)

[elasticmachine]

Pinging @elastic/es-search (:Search/Percolator)

[ruke47]

Additional details:

If I create the percolator, and then add a "message": { "type": "text"} mapping to the index, the percolate query still fails. If I add the mapping to the index before creating the percolator, the percolate query will succeed.

[avivunitq]

I am having the same issue in v7.9.1 as well as 7.10.2! Is there any update on this bug?