Rich-text editor removes newline following horizontal rule, turning it into four dashes

When you insert a horizontal line (something like <hr>) using the WYSIWYG editor, and then right behind it an emoticon, the horizontal rule is not displayed correctly. Instead, the four hyphens and the emoticon are shown on one line. See attachments.

How to reproduce:

• insert a horizontal line
• immediately after that insert an emoticon
• save the page
• resulting code is displayed incorrectly.