Understanding Elasticsearch TF/IDF score in 6.8

[AV7]

Hi,
I was looking at the lucene TF/IDF practical scoring function vs what is reported in the ES explain API. For ES 6.8, it used lucence 7.x for calculating the TF/IDF score which has a squared IDF term

However, when I look at the explain results for 6.8, I just see one idf term instead of a squared idf.

Am I missing something or is there a missing idf term in explain?

[Christian_Dahlqvist]

By default I believe Elasticsearch 6.8 uses BM25 instead of TF/IDF. What does your query look like?

[AV7]

I have set the similarity module to classic i.e. TF/IDF

[AV7]

The query has just a single term.