Files are disappearing without any obvious reason, they seem to be deleted. The client itself doesn’t delete files on his own, therefore the most likely reason is that the files were deleted by an other member of the folder on an other device.
The client moves deleted files into its internal file archive by default, so the files are actually not deleted. When displaying only deleted files in the internal file browser of the client, you can also see from which machine or from which user the file was last changed (deleted), so you can check why it was deleted there.
As soon as you restore a file or multiple files from the archive those changes will be propagated to other clients as well, so the file(s) will be available on all devices again. If the files were backed up online, they can be restored using cloud web interface as well.