No, it is not.
Discourse, like many others, considers everything that comes after a simple newline as part of the paragraph.
Only when another newline is inserted, the interpreter can see it as something independent (a separator or whatever).
It’s not a bug, it’s a feature.
That is why you get the realtime preview, so that you can see how your message will look afterwards.
ernolf
I once again, urge you to open an issue on GitHub and discuss it directly with the developer community, as this is a fairly complex topic also involving a number of security considerations.
…and regarding the Markdown issue: (one last time 
)
I’m afraid we have to admit that we were wrong. It’s an official syntax for second-level headers… Daring Fireball: Markdown Syntax Documentation