Auto backup not working reliably

I’m getting inconsistent behaviour from the Auto backup preference setting.
I’m clearly seeing backup files created for some of my projects, but definitely not for all of them.

There seems to be no common trigger that I can see that causes backups to stop being made.
Curious to know if others have noticed this too.

Can you reproduce it for a single project so that backup settings are defined but no backup is created?

Never mind. User error. I got confused with the setting in Preferences.
If you create new projects when the Auto backup in Preferences is off, it will remain off for those created projects. I didn’t know you had to enable/disable it on a per project basis for existing projects. I’m used to global back up settings, not per project.