I can at least successfully reproduce it when I edit an *existing* post, like this one.
Here are the steps to reproduce:

1. Create a message with a code block with two HTML breaks.
2. Post the message
3. Click Edit to edit the message
4. Make a change to your message, and make a spelling mistake so you get to see the Edit Reply button. Click the button to return to your message.
5. Now your breaks will be changed to Snitz code...

Imar