Intriguing. Yeah, this is definitely limited only to Dev, as Production works as intended. By far, the easiest method to fix this for you, is to modify line 348 from this: .replace(/\]\_\[/g, "<hr style='border: 0; height: 0; border-top: 1px solid rgba(0, 0, 0, 0.1); border-bottom: 1px solid rgba(255, 255, 255, 0.3); margin-bottom: 3px; margin-top: 3px;'/>"
To something like this: .replace(/\~\_/g, "<hr style='border: 0; height: 0; border-top: 1px solid rgba(0, 0, 0, 0.1); border-bottom: 1px solid rgba(255, 255, 255, 0.3); margin-bottom: 3px; margin-top: 3px;'/>")
Ultimately, that will simply mean that instead of using ]_[ for a horizontal rule, you would use ~_.