### Environment:
Elasticsearch version:
```
{
    "number" : "7.15.0",
    "build_flavor" : "default",
    "build_type" : "zip",
    "build_hash" : "79d65f6e357953a5b3cbcc5e2c7c21073d89aa29",
    "build_date" : "2021-09-16T03:05:29.143308416Z",
    "build_snapshot" : false,
    "lucene_version" : "8.9.0",
    "minimum_wire_compatibility_version" : "6.8.0",
    "minimum_index_compatibility_version" : "6.0.0-beta1"
}
```
Plugins installed: []
JVM version :1.8
OS version :Windows 10

### Steps to reproduce:
```
PUT test_plain_highlighter 
{}

POST /test_plain_highlighter/_doc/1
{
  "description": [
    "Lorem Ipsum string Generator that helps to create dummy text for all layout needs.",
    "It has roots in a piece of classical Latin literature from 45 BC, making it search string over 2000 years old."
  ]
}

POST /test_plain_highlighter/_search
{
  "highlight": {
    "order": "score",
    "type": "plain",
    "fields": {
      "*": {}
    },
    "fragment_size": 50
  },
  "query": {
    "match": {
      "description": "search string"
    }
  },
  "_source": ""
}
```

### RECEIVED Highlighted Fragment in response:
```
[
   "Lorem Ipsum <em>string</em> Generator that helps to create",
   " literature from 45 BC, making it <em>search</em> <em>string</em> over 2000 years old."
]
```
### EXPECTED Highlighted Fragment:
```
[
   " literature from 45 BC, making it <em>search</em> <em>string</em> over 2000 years old.",
   "Lorem Ipsum <em>string</em> Generator that helps to create"
]
```
server/src/main/java/org/elasticsearch/search/fetch/subphase/highlight/PlainHighlighter.java
private static final String CACHE_KEY = "highlight-plain";
    @Override
    public HighlightField highlight(FieldHighlightContext fieldContext) throws IOException {
        SearchHighlightContext.Field field = fieldContext.field;
        FetchContext context = fieldContext.context;
        FetchSubPhase.HitContext hitContext = fieldContext.hitContext;
        MappedFieldType fieldType = fieldContext.fieldType;
        Encoder encoder = field.fieldOptions().encoder().equals("html") ? HighlightUtils.Encoders.HTML : HighlightUtils.Encoders.DEFAULT;
        if (fieldContext.cache.containsKey(CACHE_KEY) == false) {
            fieldContext.cache.put(CACHE_KEY, new HashMap<>());
        }
        @SuppressWarnings("unchecked")
        Map<MappedFieldType, org.apache.lucene.search.highlight.Highlighter> cache = (Map<
            MappedFieldType,
            org.apache.lucene.search.highlight.Highlighter>) fieldContext.cache.get(CACHE_KEY);
        org.apache.lucene.search.highlight.Highlighter entry = cache.get(fieldType);
        if (entry == null) {
            QueryScorer queryScorer = new CustomQueryScorer(
                fieldContext.query,
                field.fieldOptions().requireFieldMatch() ? fieldType.name() : null
            );
            queryScorer.setExpandMultiTermQuery(true);
            Fragmenter fragmenter;
            if (field.fieldOptions().numberOfFragments() == 0) {
                fragmenter = new NullFragmenter();
            } else if (field.fieldOptions().fragmenter() == null) {