Mike at Bombich Software, the makers of the excellent Carbon Copy Cloner, writing about a change in the newest version of Catalina:
Early last week we discovered an APFS filesystem bug in a beta of macOS 10.15.5. The technical details of the bug are laid out below, but the short version is that we’re no longer able to use our own file copier to establish an initial bootable backup of a macOS Catalina System volume. To be very clear – existing backups are unaffected, and this has no effect on CCC’s ability to preserve your data, nor any effect on the integrity of the filesystems on your startup disk or your backup disk. The impact of this bug is limited to the initial creation of a bootable backup.
Check out the full post for the technical details, but in short, it’s hard to tell if this is a bug or a change in Apple’s security policy concerning the Mac. If it’s a bug, I have questions about what sort of change could impact this toward the end of an OS’ active development, and if it’s a change, it should have been documented when it first shipped in the beta.