Update: SuperDuper developer Dave Nanian reports that Apple has fixed the issue in macOS 15.3.
One or more backup bugs in macOS 15.2 Sequoia is affecting Apple’s own Time Machine utility, as well as third-party apps SuperDuper and CarbonCopyCloner. Initially the problem appeared to affect bootable backups only, but it now appears that it is either more general than this, or there is more than one bug affecting Mac backups …