Still use what @MusicalBox has shown, but first you need to set your Video Mode to something like HD1080p 30fps.
When you leave the Video Mode in “Automatic” it sets to the first video specifications opened. In the future when editing with Shotcut, always set the Video Mode before opening any video. Shotcut does remember the last Video Mode set, so you if always want to use that same setting, you only need to set it once.