Files are disappearing without any obvious reason, they seem to be deleted.
The Client itself does not delete files, therefore the most likely reason is that the files were deleted by another member of the folder on another device.
The Client moves deleted files into its internal file archive by default, so the files are actually not deleted.
When displaying deleted files in the internal file browser of the Client, you can see from which machine or from which user the file was last changed (deleted), so you can check who deleted it.
As soon as you restore a file(s) from the archive those changes will be propagated to other devices which have the Clients installed, so the file(s) will be available on all devices again. If the files were backed up online, they can be restored using the web portal as well.