I thought at first that this was a sync issue, but the fact that the files can be opened suggest that the files are syncing, but just not being displayed. All created at around the same time, on the same computer. For example file-1 and file-4 show in explorer, but file-2 and file-3 do not.
This is seemingly random some files created at the same time appear and some do not. If I close it again without saving, it does not reappear. If I save the file, it will reappear in Windows Explorer. The strange thing is that the files are actually there If I open excel, for example, and select the file from the recently opened files list, the file opens. I can view and access files via my phone and via dropbox online, but some files are not visible in Windows Explorer (Win 10).
