If you get unexpected black bars when you export your video, or your video resolution is different than what you set in the Export tab, this is the reason why.

You likely have set you video settings to Automatic. When you open the first video clip in Shotcut, Shotcut uses that video clip to set the video resolution.

One way to over come this is to make sure your first clip is same as the final video resolution. The other way is to manually set the video settings at the beginning of a new video project.

This video below explains it visually how to set this.

Hope this answer helps.

My Shotcut training course covers all these step by step and is probably a faster way to learn Shotcut.