### 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
fragmenter = new SimpleSpanFragmenter(queryScorer, field.fieldOptions().fragmentCharSize());
            } else if ("simple".equals(field.fieldOptions().fragmenter())) {
                fragmenter = new SimpleFragmenter(field.fieldOptions().fragmentCharSize());
            } else if ("span".equals(field.fieldOptions().fragmenter())) {
                fragmenter = new SimpleSpanFragmenter(queryScorer, field.fieldOptions().fragmentCharSize());
            } else {
                throw new IllegalArgumentException(
                    "unknown fragmenter option [" + field.fieldOptions().fragmenter() + "] for the field [" + fieldContext.fieldName + "]"
                );
            }
            Formatter formatter = new SimpleHTMLFormatter(field.fieldOptions().preTags()[0], field.fieldOptions().postTags()[0]);
            entry = new org.apache.lucene.search.highlight.Highlighter(formatter, encoder, queryScorer);
            entry.setTextFragmenter(fragmenter);
            
            entry.setMaxDocCharsToAnalyze(Integer.MAX_VALUE);
            cache.put(fieldType, entry);
        }
        
        int numberOfFragments = field.fieldOptions().numberOfFragments() == 0 ? 1 : field.fieldOptions().numberOfFragments();
        ArrayList<TextFragment> fragsList = new ArrayList<>();
        List<Object> textsToHighlight;
        final int maxAnalyzedOffset = context.getSearchExecutionContext().getIndexSettings().getHighlightMaxAnalyzedOffset();
        Integer queryMaxAnalyzedOffset = fieldContext.field.fieldOptions().maxAnalyzedOffset();
        Analyzer analyzer = wrapAnalyzer(
            context.getSearchExecutionContext().getIndexAnalyzer(f -> Lucene.KEYWORD_ANALYZER),
            queryMaxAnalyzedOffset
        );
        textsToHighlight = HighlightUtils.loadFieldValues(
            fieldType,
            context.getSearchExecutionContext(),