From your description, it sounds like this would have Slack's problem (which resulted in one of the most upvoted HN threads of all time, https://news.ycombinator.com/item?id=21589647). I've tried Googling around a bit to find an example of Jira Cloud's Markdown environment, but couldn't find one. (I am certainly biased, though, by my strong loathing of Jira.)