I am not able to replicate this issue locally. Can you try to do it on our demo website - http://demo.sitefinity.com/ and let me know the steps you use and browser you used for the test.
In FireFox or/and Internet Explorer when you hit the "Enter" this generates <p> or <br> tags.
Check the css classes that apply when you publish the page. It is possible that you set some rules from your custom stylesheets.
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