You mean during a single session? I.e. when reloading a directory, the content changes each time?
This indicates that all files are still there, but the directory listing doesn’t seem to work.
Can you please share some details? We don’t know your setup and it is hard to find the cause of your problem without any information.
What software is involved in which versions on what operating systems? How many devices are involved? What are your sync settings? I don’t know Insync, does it sync files or provide transparent access (like Google’s File Stream or OneDrive’s Files on Demand)?
Last but not least, check your log files for warnings or errors around the time you experienced the disappearing files.
Same goes for Sanitizer: What in Sanitizer’s log file? The explanation what happened to your files is probably in there.