There is already Insider feedback that OneDrive is not syncing Shared/Added folders, but in fact is is actually deleting the files on the PC. As I keep backups of my backup I can deal with this, but I won't be using OneDrive again until I know this is fixed.
It is still working OK in Win7 (another reason I am keeping Win10 as a beta system).
I am now at another PC in a different location. It was running Win7 when there was a similar symptom. I stopped OneDrive when I saw files being deleted from the same Shared/Added folder: but found them in the Recycle bin. I restored them, restarted OneDrive and it continued normally, nothing else was deleted. Very odd. Possibly it had decided the PC versions of the files needed refreshing from the server and it was deleting them before downloading to refresh. But as the files were not changed, and there are 30GB of them, this is unwanted behaviour.
Whatever was happening, it's not a build 14257 issue as this was seen in Win7: but still specific to Shared/Added folders.