CKEditor automatically deletes content in some circumstances

Steps to reproduce:

1. Create a new page (P1) with the following content:

   {{html}}
   <iframe width="560" height="315" src="https://www.youtube.com/embed/dQw4w9WgXcQ" frameborder="0" allowfullscreen></iframe>
   
   {{/html}}
   
   //this is my text in italic//

   (intentionally leave an empty line before closing the HTML macro)

2. Edit P1 using CKEditor
3. Save P1 -> the content changes to

   {{html}}
   <iframe width="560" height="315" src="https://www.youtube.com/embed/dQw4w9WgXcQ" frameborder="0" allowfullscreen></iframe>
   
   {{/html}}

The line "//this is my text in italic//" was deleted without any input from the user.