Not sure if anybody else is having issues with "\overline" just not showing up now that mathjax was upgraded to v3 (at least that's what I'm blaming it on). I noticed that if I switch to the tex-svg.js MathJax output (by editting the html files), overlines show up again.
I couldn't really find anyone else noticing this via a quick google search, so I submitted a mathjax issue on this, we'll see what they say. Perhaps it's just my particular browser (chrome on linux) or some other issue?
Best,
Jiri