API URL: https://en.wikipedia.org/w/api.php?action=query&list=search&srsearch=intitle%3A%22donkeys%22&srlimit=500&srnamespace=0&srprop=size%7Cwordcount%7Ctimestamp%7Credirecttitle&format=json&origin=*&srsort=create_timestamp_asc
For the query [intitle:"donkeys"], the page "Democratic Party (United States)" is one of the results. It should not be listed, since no redirects to that page include plural "donkeys" (list of redirects), and according to Help:CirrusSearch#Intitle_and_incategory, quotes turn off stemming.
Another potential issue: the result item is missing a redirecttitle: field. I suspect the result is listed because of redirects with "donkey" in the title, but the lack of redirecttitle: makes it look (to a tool we're developing) like it's a non-redirect result.
This isn't an isolated example: that same query includes "John Simpson Kirkpatrick", "Buridan's ass", "Evolution of the horse", which similarly have no redirecttitle: field (though I haven't verified they don't have redirects with "donkeys" in the title). Another example: "Prometheus" as first result for intitle:"eagles".