File finshes downloading, then tribler deletes it, starts again

The file finishes downloading, the video works.
The next time I open Tribler, it starts downloading from the beginning i.e. it DELETES THE FILE.
Other torrent programs I have used have done this as well.
The result is that I feel I need to immediately delete the torrent upon completion without seeding for fear of losing the files.
Why do torrent clients do this? What’s so hard about checking the download location and noticing that the file is already there?
Can you please stop overwriting files without warning!?

I think I know what has happened. Although the torrent had supposedly completed the subtitle file was missing. Recheck got stuck at 99% and so I unchecked the video file so only the subtitle file would download. By unchecking the video file Tribler deleted it.
These file check boxes are there to tell the client what to download and in my opinion should not be used to delete files, especially without warning. If a file is already downloaded unchecking the box should have no effect.