I noticed an odd issue when trying to edit an old post.

I clicked on the edit button and made some changes using the standard plain-text editor. Note the newlines:

Then, when I either post or switch to the preview view, the newlines are gone:

Interesting note: once I go into the preview, I get a WYSIWYG editor instead of a plain-text one (which shows the formatting instead of the tags), and if I make newlines there, they work.

I guess this happens because I tried changing the editor style to "Enhanced Interface - Full WYSIWYG Editing" a while back.
Since I don't actually see the WYSIWYG editor, I forgot about the change and just left it there.
Changing it back to "Standard Editor - Extra formatting controls" makes the issue no longer manifest.