Mou handles it as I'd expect:

But Github doesn't:

what the crap

_what _the* crap*

Online Kramdown Editor gets it:

These nested markups are always problematic in Markdown. There are discussions at the issue tracker of Hoedown (which is MacDown’s main Markdown backend) about similar issues, too, like hoedown/hoedown#78 and hoedown/hoedown#138. There are people working on it, but this will require quite some rewriting of logic, which takes time.

Since this only happens with the renderer (the syntax highlighter works as expect), I’ll close this so discussion can be centralised at the aforementioned Hoedown issues. It does not mean that we won’t fix this.