I'm not sure if this is related to the ParserFunctions plugin, but all research I've exhausted points to this. I'm not sure how to fix this, however. Hopefully someone can help.
Out of nowhere, when a term is searched that has an ampersand (&) or an apostrophe ('), the URL returns the article double-encoded. i.e. a term with an "&" is selected from the search box drop-down and instead of the standard %26, it returns as %2526, thus producing a "Bad title" error. With apostrophes in the term, it transcodes to %2527.
I'm using MediaWiki version 1.31.