I can't think of what it must be.
This post was done the same way I always do it.
I have typed short lines to represent very small paragraphs, and after each of them, I hit Enter twice.
When I only hit it once, it looks like the spacing between the line above and this line.
Then I hit Enter twice again, and it spaces out like this.
Something is different, maybe it is a setting in your operating system, but, using either Enter key is working for me.
I am not the person to make suggestions on fixing Windows, I am an old DOS/text based computer dinosaur, I only tolerate Windows and Mac because no one uses AppleSoft and PC-DOS anymore, hah.
Rex