qBittorrent official forums

What is almost driving me mad is that: As I was downloading the files(which some of them incomplete or complete, maybe took days and weeks even months!); then one day several files are missing or disapeared(from the list), or restarting  the completed files(still in the list with no progress)!
sometimes I was lucky that the completed downloaded files which appeared missing from the list still in the PC, sometime not.
Why? and is this a coincidence?

This sounds really weird.
- What Linux distro are you using?
- What filesystem are you using? Is it a local filesystem?
- You have the correct permissions?
- Are you using the latest qBittorrent version?