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" ([[ https://dispenser.info.tm/~dispenser/cgi-bin/rdcheck.py?page=Democratic_Party_(United_States) | list of redirects ]]), and according to [[ https://www.mediawiki.org/wiki/Help:CirrusSearch#Intitle_and_incategory | 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 [[ https://gitlab.com/falsifian/wikipedia-title-search | 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".