The point of the stubs is so you don’t have to know which cloud files are actually on your device at any given moment, because they will be fetched automatically. Marking files as “always local” is a niche feature, and in any case has nothing to do with whether you want those files backed up.

To have this thing you’re not supposed to need to worry about affect whether your files got backed up is exactly the problem here. The goal is to back up your files, whether they’re in the cloud or not.

I sympathize with Backblaze’s problem with their file change monitor, but then they should considee implementing connectors for OneDrive, Dropbox, etc. and back up files directly from the cloud.