Why would the default Ingest Pipeline in version 8.4 not run?
There's not too much information about the conditions to which the pipeline would run, it just says that every document that is found by the crawler should go through this pipeline..
Could you tell me more information about your setup? From what i've read, you have an app search engine with a web crawler configured to your domain. The app search crawler has setup a default ingest pipeline primary focused on documents, to deal with binary content extraction from incoming documents and run when there is a presence of an _attachment field.
Right now you're seeing no activity within your pipeline and questioning when does it run? I believe the pipeline should run always but the processors in pipeline will execute based on their own set of conditions.
Have you invoked a crawl yet through the UI for your domain?
In the release notes you can reference the section:
" * App Search web crawler support for binary content extraction is now generally available. Crawl binary content such as PDF and Office documents. Note that only binary content (not HTML) is sent into the app_search_crawler pipeline, whereas the Elastic web crawler sends all indexed documents into the ent_search_crawler pipeline. See the breaking changes section if you have modified the App Search web crawler ingest pipeline settings."
This works for the Web Crawler that you can access and use in 8.4 (Top level navigation -> Content -> Indices -> Create Index) for Enterprise Search. You can then connect that web crawler to App Search capabilities by using Elasticsearch index engines (beta) | Elastic App Search Documentation [8.4] | Elastic to the created index. I'm super interested in your feedback on how these different crawlers are working for you - so if you'd like to do a quick user feedback session please let me know!