When a directory in the set of source directories is missing, the backup fails and does not proceed.
This happened to me when an application (on macOS that is technically a directory) that was explicitly added to the backup was deleted. From the moment on that application was deleted, the backup failed.
On macOS at least the behaviour is undesirable. But isn’t it undesirable at large? After all, when I accidentally delete a directory on the backed up system that I have explicitly added to the backup, the backup should still work for the rest. I would only fail if there is no source to backup at all.