There are no CCC nor macOS bugs here, it’s normal and correct for CCC to indicate that the source volume is not available when you migrate your data to a new Mac. CCC isn’t making any assumptions here either – indeed, the old Mac’s “Macintosh HD” volume is not available on the new Mac, the new Mac has its own “Macintosh HD” volume. CCC doesn’t rely on the volume name alone for identification, it uses the volume’s universally unique identifier.
So, I needed to rename the Data volume via Disk Utility as ‘iMac 5K Disk - Data’. Once I did that, I was able to create a proper clone of the boot drive.
Renaming the Data volume has no effect on CCC’s identification of the source or destination, CCC never considers the name of the Data volume (as you suggested, CCC uses volume roles alongside the UUID to identify the Data volume). Alan must have reselected the Macintosh HD volume as the source, that’s the only thing you need to do to get the task configured correctly for a new Mac.
Mike (author of CCC)