It appears to be a mismatch of video mode to source, and modified export settings.
When you modify the export settings from default, it offsets everything from the Video Mode, not your source video.
Here is how you get the video mode specifications. Automatic is set to what you first bring into Shotcut. So at this point I have no idea what your specifications are.
To correct the situation, set the Video Mode, and go to the export tab, click on default, then click Reset before exporting again.
