What this conundrum reveals is that mindlessly trawling Stack Overflow like I did can result in brittle, potentially dangerous code. The XML specification defines five "predefined entities" representing special characters, and requires that all XML processors honor them.
The decimal equivalent of the code point is then shown in parentheses. The entity must either be predefined built into the markup language or explicitly declared in a Document Type Definition DTD.
However, due to finagling elsewhere in a system, there might not be any red flags raised until it comes time to parse it. XML also allows other named entities of any size to be defined on a per-document basis.
The entities can be explicitly declared in a DTD, as well, but if this is done, the replacement text must be the same as the built-in definitions.
The American Mathematical Society also contributed entities for mathematical characters. The system which directly used the XML file was a black box and the config scraper from which I was pulling the XML file for a tangental purpose was also a black box.