The same thing happened to me, but I didn't use the Common App. I used the school's regular online application, and whenever I would leave the page of my essays, it would bring up an image of them and it looked like the formatting was perfect. Otherwise there was no option to preview the application (at least that I know of, I could have been mistaken.) After I submitted it and I had the opportunity to look at the finished product as a PDF, I saw that the essays looked like a giant block of text. After consulting my guidance counselor, she said it wasn't a big deal and I shouldn't worry about it.
Honestly, although it may be frustrating to an admissions officer, I doubt they'd disregard your essay because of it.