Code that is pasted into another code block loses all its formatting

When the contents of a code block is copy&pasted into another code block, then it loses all its formatting. Please see the attached screenshot for an example.

Steps to reproduce:

1. Create a blank page.
2. Insert a code block macro.
3. Type some formatted code with indentation etc.
4. Create another code block macro.
5. Copy and paste the code into the second code block.
6. Now all formatting is lost and the code is inserted as a single line.

When the code is instead pasted into the normal page body, the formatting is preserved.