I suspect it is a coding problem brought about by the dramatic changes in Big Sur's internal organization and protection. Remember, at present only the data volume can be cloned, even using Apple's own ASR utility and Time Machine will only back up the data volume. In retrospect, that shouldn't be a surprise because iOS and iPadOS never backed up the system either.
If we knew what it was we were doing, it wouldn't be called research, would it?
