Jay said:
Hold down the Shift key when you press enter/return and you'll get a single new line.

Thank you.
I'm happy to now know a work-around for this bug.

Now how to turn off this WYSIWYG mode and see the markup tags?

I refuse to accept the notion that a thing is 'right' simply because that's the way Microsoft Word does it.
Within the context of editing or word processing software, I believe the Enter key or whichever key occupies that patch of real estate should retain backward compatibility with IBM's Selectric typewriter. While that key could be user-configured to double-space with one press, IBM never intended it as a "new paragraph" key.