Elasticsearch combined_fields query with synonym_graph token filter - elasticsearch

Question

I'm trying to use a combined_fields query with a synonym_graph search-time token filter in Elasticsearch. When I query for a multi-term phrase in my synonym file, Elasticsearch seems to unconfigurably switch from "or logic" to "and logic" between my original terms. Here's an example Elasticsearch query that has been exaggerated for demonstration purposes:
GET /products/_search
{
"query": {
"bool": {
"should": [
{
"combined_fields": {
"query": "boxes other rectangle hinged lid hook cutout",
"operator": "or",
"minimum_should_match": 1,
"fields": [
"productTitle^9",
"fullDescription^5"
],
"auto_generate_synonyms_phrase_query": false
}
}
]
}
}
}
When I submit the query on my index with an empty synonyms.txt file, it returns >1000 hits. As expected, the top hits contain all or many of the terms in the query, and the result set is composed of all documents that contain any of the terms. However, when I add this line to the synonyms.txt file:
black spigot, boxes other rectangle hinged lid hook cutout
the query only returns 4 hits. These hits either contain all of the terms in my query across the queried fields, or both the terms "black" and "spigot".
My conclusion is that presence of the phrase in the synonyms file is influencing how the "non-synonym-replaced" phrase is being searched for. This seems counterintuitive - adding a phrase to the synonyms file should only possibly increase the number of results that a search for that exact phrase produces, right?
Does anyone know what I'm doing incorrectly, or if my expectations are reliant upon some fundamental misunderstanding of how Elasticsearch works? I observe the same behavior when I use a multi-match query or an array of match queries, and I've tried every combination of query options that I reasonably think might resolve the problem.
For reference, here is my analyzer configuration:
"analysis": {
"analyzer": {
"indexAnalyzer": {
"type": "custom",
"tokenizer": "standard",
"filter": [
"lowercase",
"porter_stem",
"stop"
]
},
"searchAnalyzer": {
"type": "custom",
"tokenizer": "standard",
"filter": [
"lowercase",
"porter_stem",
"stop",
"productSynonym"
]
}
},
"filter": {
"productSynonym": {
"type": "synonym_graph",
"synonyms_path": "analysis/synonyms.txt"
}
}
}