What does ‘stick’ mean? Changes only affect future backups, unlike on some other backup programs where deselection purges data. The PURGE command is a Duplicati do-it-yourself (for example with Commandline).
Was this trying to help the ‘stick’ or the error? Either way it may be because it doesn’t change existing things. The DELETE command is a way to delete entire versions of the file view. In the job, retention sets up deletes.
Any more test clues, anyone? I tested unchecking a source directory that had been backed up. All still works. There’s also a theory (pointing to this article actually) that privilege changes are involved in producing this…