Elasticsearch ngrams (Partial mappin)

Using Neos & Flow
1

Dear Community,
I’m trying to add partial searching on my site. I found “NGrams” [https://www.elastic.co/guide/en/elasticsearch/guide/current/ngrams-compound-words.html]. However, it doesn’t work.

Flowpack.SearchPlugin/Configuration/Settings.yaml

  ElasticSearch:
    indexes:
      default:
        searchindex:
          analysis:
            filter:
              autocompleteFilter:
                max_shingle_size: 5
                min_shingle_size: 2
                type: 'shingle'
              trigrams_filter:
                type: 'ngram'
                min_gram: 3,
                max_gram: 3
            analyzer:
              autocomplete:
                filter: [ 'lowercase', 'autocompleteFilter' ]
                char_filter: [ 'html_strip' ]
                type: 'custom'
                tokenizer: 'standard'

              trigrams:
                type: 'custom'
                tokenizer: 'standard'
                filter: ['lowercase', 'trigrams_filter']

and I add the analyser in my own NodeTypes.yaml (Packages/Sites/vendor/configuratin/NodeTypes.yaml)

properties:
    'chapterDescription':
      type: string
      search:
        fulltextExtractor: '${Indexing.extractHtmlTags(value)}'
        elasticSearchMapping:
          type: string
          analyzer: trigrams

When I check the PUT Query, it looks like the same as the one in the Elasticsearch documentation. But the search doesn’t list anything when I search for >3 letters which are existing in chapterDescription.

Ex.: Grund­stücks­ver­kehrs­ge­neh­mi­gungs­zu­stän­dig­keits­über­tra­gungs­ver­ord­nung
Searchterm: ver­kehr
Result : 0, nothing.

Has anyone faced a problem like this or has a hint to solve the problem?
Thank you.

Best,
Kemal