Bombich describes the bug at length in the blog article and how it was discovered.
“Earlier this week I noticed that an APFS-formatted sparsebundle disk image volume showed ample free space, despite that the underlying disk was completely full. Curious, I copied a video file to the disk image volume to see what would happen. The whole file copied without error! I opened the file, verified that the video played back start to finish, checksummed the file – as far as I could tell, the file was intact and whole on the disk image. When I unmounted and remounted the disk image, however, the video was corrupted. If you’ve ever lost data, you know the kick-in-the-gut feeling that would have ensued. Thankfully, I was just running some tests and the file that disappeared was just test data. Taking a closer look, I discovered two bugs in macOS’s “diskimages-helper” service that lead to this result.”
Bombich states that the issue isn’t likely to affect a large amount of CCC users. However, he added, “Until Apple issues a macOS update that resolves this problem, we’re dropping support for APFS-formatted disk images.”
You can read more about the APFS bug at Carbon Copy Cloner’s official blog: bombich.com/blog/2018/02/15/macos-may-lose-data-on-apfs-formatted-disk-images