I tracked this down: While Discourse is running MathJax v2.7.5, the current version is v.3.2.2, which is what we run on our site. Since the config file is handled differently between v2 and v3, our config isn’t getting applied to what Discourse displays on the embed on our site. Among other things, that means that the single dollar signs aren’t being recognized as indicating inline math, since that’s not the default in MathJax (or LaTeX, for that matter), and so must be specified via local config.
MathJax is now in the process of rolling out v4.
Curious @sam as to whether there are plans to update the version of MathJax Discourse is using?
Really appreciate that Discourse has the capability to display beautiful math via MathJax!