I have a different example of Google not finding a page with "" that ought to be found. Assume I want to find https://board.s9y.org/viewtopic.php?t=18685, which is on a ~20 year old phpBB forum that is certainly indexed. The post is from 2012 after all. And I take one sentence from there and search for it, with quotes:

"Hast du besondere s9y eventplugins installiert?"

This leads to https://www.google.de/search?q=%22Hast+du+besondere+s9y+even..., which so far has only one search result, to http://sw-guide.de/2006-11/serendipity-plugins-seitenleisten.... Which by the way does not contain the target sentence, but that might be because the search moved over into non-verbatim mode.

If I search without the quotes I get more results, but I think not the target thread.

Admittedly, this is not "" not working by showing pages that do not contain the target keyword. But it is "" failing by the search not properly searching its own index.

What's going on there? Why is the index of a rather stale 20 year old forum not complete? The site is indexed in general.