I’m glad you got it working satisfactorily. Depending on how much more you want to work on improving anything, I’d like to at least try to clarify the situation, and maybe get deeper technically. Starting easier:
Although I hate to suggest breaking what’s been fixed, it would be nice to know if versions without Smart Sync could hit this, for example if cache can break even if Smart Sync isn’t available to the final user file.
OneDrive Files On-Demand had an earlier case of this issue, and Duplicati did some work like Fix 2829: Reparse points aren’t always symlinks #3014 to fix issue With Windows Fall Creators Update OneDriver Folder cannot be backupped any longer #2829. There were also some forum reports about this, I believe.
I almost never use OneDrive, so am not quite sure where things stand now in terms of what’s backed up. Regardless, it might be possible to get Dropbox to reach parity with whatever it is that OneDrive’s gotten. This wouldn’t be anything I would develop, but I could probably help with your research. Or we could wait. Possibly things will stay quiet on the Dropbox front for awhile. There’s ample other work to do meanwhile.
NTFS reparse point is the kind of thing this will get into. See earlier Dropbox technical article links as well. Fsutil reparsepoint is one tool that can query paths to see if some high-tech remote capability might exist. –exclude-files-attributes can exclude reparse points, but I’d worry that it might exclude desirable varieties.
That’s pretty deep, so no problems if you don’t want to dig in. To simply close this for now, I’d appreciate a note on what the SharePoint issue turned out to be (if you know), and perhaps a bit more OneDrive detail.
Thanks!