For a vertical video, you need to set a Vertical Video Mode.
Settings → Video Mode → Non Broadcast → Vertical 30 or 60 fps
Once the Video Mode is set, then you can open your media into Shotcut.
No need to go into Advanced in Export when you set the correct video mode.
Default Video Mode is “Automatic”.
So you started with a 1920x1080 video mode at 25fps.