In addition, immediately after the upload to YouTube completes, it only offers a low resolution version until more resolutions, bitrates, and codecs are done. You should ensure the resolution you export from Shotcut is at least as high as the source using Settings > Video Mode (not Export > Advanced > Video > Resolution).
Did you rotate your video to make it upright? Depending on how you did that it will lose clarity. For your video, I open it, change Properties > Video > Rotation to 90. Next, choose Settings > Video Mode > Automatic. Finally, I export using the default settings and upload to YouTube. How does this compare with what you experienced? (Immediately after upload this is only 135x240 resolution even though the export is 720x1280! Now, after several minutes I see a 720p50 option in the YouTube quality selector, which is a little misleading because YouTube’s “Stats for nerds” shows “720x1280@50.”)
