Try opening the “Search tools” menu in the results page and selecting “Verbatim”... I think that makes “literal” searches work, because otherwise Google uses synonyms and language analysis to attempt to match your search to results.
The query parameter google adds for verbatim matches is tbs=li, so you can append that to the url, like
https://www.google.com/search?q="somestring"&tbs=li
That too has consistently failed between 2007 and late 2020.
Now it just fails inconsistently (i.e. it sometimes works).
> “search term”
> Force an exact-match search. Use this to refine results for ambiguous searches, or to exclude synonyms when searching for single words.
Author claimed they tested all of the operators, but this one in particular I can confirm haven't worked reliably for me for over a decade and across a number of machines and accounts.
Some times it can be traced down to things like me searching for "foo bar" and Google thinking
> He said foo. Bar immediately replied [...]
is a valid match which is kind of understandable.
But most of the time I can't really see why the page shows up at all. No trace when searching in the page nor in the page source.