I have the latest installer and I can reproduce it when zooming in.
With the \bar the line stays visible. Strange that it disappears for \overline
But on 100% they both look good:
Recently there was also a bug report with lines disappearing when using brackets when zoomed in: Obsidian improperly renders large brackets in LaTeX math mode
Hopefully the devs can take a look if Mathjax is correctly implemented for Obsidian when zooming in or that it is a bug on the Mathjax side.