We have a synonym filter configured in the analyzer used for queries (but not in the analyzer used for indexing).
If I do a Match query on a single search term with a minShouldMatch value of "2<75%", the query will not return results if the single term has multiple synonyms. Seems like the expanded synonym terms are affecting the minShouldMatch calculation.
If I don't include the synonym filter in the query analyzer it works, or if I change minShouldMatch to "1<75% 2<75%" it works.
More info: This only seems to happen using a Common Terms query - a Match query works OK. This is on ElasticSearch 5.4,