The search engine is only considering whitespace delimited text when searching for exact results.
In other words, searching for exact matches requires you to include punctuation. In your first example, there is no literal
what have you tried sentence without punctuation in the question text. There are only forms with quotes around them:
"What have you tried?"
"What have you tried"
I have not been able to figure out how to search for an exact phrase with quotes; there seems to be no way to escape literal quotes available.
But you can search for another phrase that includes punctuation:
This is generally a good thing,
"This is generally a good thing" is:q fails, but if you add the comma and search for
"This is generally a good thing," is:q then suddenly there is a match.
I am not that familiar with elasticsearch / Lucene configuration, but if it is possible to configure the index to not include punctuation when searching for exact phrases (while at the same time allowing you to match exact punctuation too), then this is a bug in how the search engine on Stack Exchange is configured.