Pasting text into Code Macro from a template erases the entire code block

When creating a page via a template that contains a code block with placeholder text, pasting text to replace the placeholder can result in the entire code block disappearing.

Steps to reproduce:

1. Create a template with a code macro and some place holder text inside
2. Create a page using this template
3. Highlight the place holder text
4. Copy and paste some text into the code block, replacing the entirety of the placeholder text

Expected behavior:
The code block contains the new text

Actual behavior:
The code block disappears entirely, leaving only the text in the editor

Additional notes:
This is also reproducible using code blocks inserted via "Insert Wiki Markup".

This behavior is NOT reproducible for code blocks inserted directly (the "normal" way) into the Confluence 4 editor