Has anyone noticed that stemming stops working when adding the .UsingSynonyms() feature to a search request?

Example:

var query = "Exhausts";
var result = SearchClient.Instance.Search(Language.English)
  .For(query)
  .UsingSynonyms()
  .InFields(x => x.DisplayName,
            x => x.PageTitle)
   .Take(100)
   .ProjectToFindListItemGroup()
   .GetResult();

results in 10 hits.. All with the word "Exhausts" in the page title.
Removing the .UsingSynonyms() results in 233 hits, including hits with both "Exhausts" and the stemmed word "exhaust" (which is the expected functionality)

I didn't think using stemming and/or synonyms were mutually exclusive?
In my perspective they shouldn't be.