If the video was recorded on a mobile device, they save all vidoes in a landscape orientation but then included a property that tells video players to rotate portrait videos by 90 degrees when they open them. Unfortunately, FlashBack does not currently support this property so will not perform the necessary rotation. 

The only suggestion we could make is to try using one of the guides in this article to re-encode your video into a true portait orientation before importing it into FlashBack - https://www.howtogeek.com/howto/14751/rotate-a-video-90-degrees-with-vlc-or-windows-live-movie-maker/