When you first use Shotcut, it starts you with “Automatic” Video Mode unless you set it manually. Video Mode settings are highly critical in any video editing software.
I’m only guessing you didn’t change the Video Mode once you converted the video to Edit Friendly within Shotcut. Go to Settings → Video Mode to change it. You probably will need to verify all edits once the new video mode is set.
Unless you have a very specific need, Default is good for an Export Preset.