C# Solr query, Boost is not working

Question

I'm using C# to query Solr for my site search. The code is fine in regard to filtering and searching for keywords in specific fields, but I can't get Boost to work. I need to use boosting so that items where the keyword is in the Title field are returned first, then keyword in the Description field, then keyword in the Content.

My search code creates a SearchQuery and adds a QueryBuilder containing a PredicateBuilder to SearchQuery.QueryBuilders. This is a simplified version of my code (took out non-relevant code)

public SearchQueryResults Search(string keywords){
     SearchQuery<KofaxSearchResultItem> queryArguments = new SearchQuery<KofaxSearchResultItem>();
     List<IPredicateBuilder<KofaxSearchResultItem>> queryBuilders = new List<IPredicateBuilder<KofaxSearchResultItem>>();
     var keywordPredicate = new ExactKeywordPredicateBuilder(keywords, _sitecoreContext);
     queryArguments.QueryBuilders = queryBuilders;
     var results = _searchManager.GetResults<KofaxSearchResultItem>(queryArguments);
     (return results after some more stuff)
}

---

class ExactKeywordPredicateBuilder : IPredicateBuilder<KofaxSearchResultItem>
{
    public string Query { get; set; }
    public List<Guid> ContentTypeFilters { get; set; }
    SearchTaxonomyUtil SearchTaxonomyUtil { get; set; }

    public ExactKeywordPredicateBuilder(string query, ISitecoreContext sitecoreContext, List<Guid> contentTypeFilters = null)
    {
        Query = query;
        ContentTypeFilters = contentTypeFilters;
        SearchTaxonomyUtil = new SearchTaxonomyUtil(sitecoreContext);
    }

    public Expression<Func<KofaxSearchResultItem, bool>> Build()
    {
        return SearchTaxonomyUtil.GetKeywordPredicate(Query, ContentTypeFilters);
    }
}

---

public Expression<Func<SearchResultItem, bool>> GetKeywordPredicate(string keyword)
{
    keyword = keyword.ToLower();
    var predicate = PredicateBuilder.True<SearchResultItem>();

    predicate = predicate.Or(r => r.Title.Contains(keyword)).Boost(1000f);
    predicate = predicate.Or(r => r.ShortPageSummary.Contains(keyword)).Boost(10f);
    predicate = predicate.Or(r => r.Content.Contains(keyword)).Boost(0.1f);

    return predicate;
}

I get the correct results in that I only get items that contain the keyword, but the boosting isn't doing anything. I'm getting keyword-in-Description results ahead of keyword-in-Title results, and if I change the boosting to make ShortPageSummary boosted higher than Title, or remove the .Boost()s altogether, the results remain the same.

I am not passing in any SortBuilder, so the sorting is whatever Solr does by default. I tried setting this:

queryArguments.SortBuilder = new GenericSortBuilder<SearchResultItem>(q => q.OrderBy(r => r["score"]));

but that returned completely jumbled results that seemed to be in worse order than they were before.

EDIT: I checked the Solr logs and the Boost is not being added to the query that is being passed to Solr.