Automatic backup not happening for one file

Scriv 3.01 in MacOS 10.13.2.
Working fine, love the improvements.
I have Automatic Backup on Close set, and other updated scrivs do get backed up to the separate drive I’ve specified.

But one old scriv project, which I did update to the new version and made a few changes to, does not back up on close. (Yes, I had made changes in the file ok. Yes it previously auto-backed-up fine when in Scriv 2. ) The file’s fine when I reopen it. The app is ok too.

Not a huge issue but I love a mystery.
No need to reply unless you want me to check something or have a tip.
Peter Mackey
Friend of Scrivener since 2007

It’s possible to disable backups or change their location on a per-project basis. Please double check the Project -> Project Settings -> Backup tab for the problem project.