The thing is moving the .bak file to a folder would break the path that is inside the .bak file.
A .bak file is just a .cpr with a renamed extension.
Easy to try, create a backup folder move a .bak file there, rename the extension to .cpr and try to load it.
Cubase can’t find the /audio folder as it is now in …/audio or whatever your OS denotes one level back.
It could be done if Cubase would write full paths to the files, but I fear that could get messy when the projects media files resides in multiple folders, as they perfectly well can be.
Personally I don’t care that much, as I limit the .bak files to 10, and change the project version number frequently, letting Cubase ad a 01 02 03 etc. at the end of the project. (Well i Save my first copy with a 01 at the end, the zero prevents the OS sorting to get messy). When I am finished for the day, I use the backup function in Cubase to move the project to a new folder again with a version number in the folder name, to indicate what is new and old. The old folder gets archived to a NAS, that runs a backup to another NAS every night.
I’m not saying everyone should be doing it this way, I started doing it this way over 15 years ago, and it works for me.
If anything can get automated in a particular workflow, like backups ,do it.
Even better make a plan on how you want to keep your files safe, print it out and hang it somewhere in view as a reminder.
I wish I had more discipline back in the day when we used real to real, those tapes have now turned to dust, before anyone got the idea to digitize them.
Sorry went a little OT here