Editor creates elements that are only styled like headings or paragraphs

The editor has a bug where elements like headings and paragraph stop being headings and paragraph semantically, but retain their styling. This breaks, among other things, the ToC macro.

How to reproduce (in Chrome):

1. Create a page. Put two headings and two paragraphs in it, in this order:

   Heading A
   Paragraph A
   Paragraph B
   Heading B
   

2. Delete the line break between Heading A and Paragraph A. Also, delete the line break between Paragraph B and Heading B, like so:

   Heading AParagraph A
   Paragraph BHeading B
   

3. The whole first line will now be marked as a heading, but the second half of it will be styled like paragraph text. Also, the whole second line will be marked as a paragraph, but the second half of it will be styled like heading text.