This is the original .mov file, it has an unusual aspect ratio, but is displayed correctly.

Above is the same video transcoded into mp4 (using handbrake, Fast 1080p30 preset). The thumbnail/tool has the wrong size, which results in a stretched preview and black bars in playback.

 

Here's an example with text, that makes the stretching of the preview thumb even more clear:

original (.mov), all correct :-)

 

transcoded as mp4, stretched preview thumb and black bars on playback:

 

a solution, have a correctly sized thumbnail (here done by hand).