Ok this was kind of tricky to find the root cause, thanks to stellar debugging tools from edge.
So it happens edge has stellar Regexp support (a pattern is emerging) and will result in this:
Edge:
Chrome:
Making this line of Discourse fail https://github.com/discourse/discourse/blob/master/app/assets/javascripts/pretty-text/engines/discourse-markdown/text-post-process.js.es6#L24 spectacularly.
I was about to write a fix for this case, but @zogstrip found a polyfill, which is probably way better as it will prevent this from happening in any other place in the app.
https://github.com/discourse/discourse/pull/7460
Thanks for reporting this @MikkoH