Spacy, Strange similarity between two sentences

The Spacy documentation for vector similarity explains the basic idea of it:
Each word has a vector representation, learned by contextual embeddings (Word2Vec), which are trained on the corpora, as explained in the documentation.

Now, the word embedding of a full sentence is simply the average over all different words. If you now have a lot of words that semantically lie in the same region (as for example filler words like "he", "was", "this", ...), and the additional vocabulary "cancels out", then you might end up with a similarity as seen in your case.

The question is rightfully what you can do about it: From my perspective, you could come up with a more complex similarity measure. As the search_doc and main_doc have additional information, like the original sentence, you could modify the vectors by a length difference penalty, or alternatively try to compare shorter pieces of the sentence, and compute pairwise similarities (then again, the question would be which parts to compare).

For now, there is no clean way to simply resolve this issue, sadly.

Spacy constructs sentence embedding by averaging the word embeddings. Since, in an ordinary sentence, there are a lot of meaningless words (called stop words), you get poor results. You can remove them like this:

search_doc = nlp("This was very strange argument between american and british person")
main_doc = nlp("He was from Japan, but a true English gentleman in my eyes, and another one of the reasons as to why I liked going to school.")

search_doc_no_stop_words = nlp(' '.join([str(t) for t in search_doc if not t.is_stop]))
main_doc_no_stop_words = nlp(' '.join([str(t) for t in main_doc if not t.is_stop]))

print(search_doc_no_stop_words.similarity(main_doc_no_stop_words))

or only keep nouns, since they have the most information:

doc_nouns = nlp(' '.join([str(t) for t in doc if t.pos_ in ['NOUN', 'PROPN']]))