Output video not matching input res or chosen aspect ratio

I took a video with my Pixel 3 XL at 3840 x 2160 resolution and imported it to Shotcut.
When I created the job, I set the job resolution at 3840 x 2160. I also used this same resolution to set the aspect ratio. And I set the frame rate as the same as the video which was 30.01 fps.

But the exported video came out at 1080 x 1440 with a frame rate of 29.52 fps.
It also came out with black bars on the sides.

I know that all phones don’t have the same aspect ratio but as I’m uploading to Tiktok with an aspect ratio of 9:16, it would be nice to have it full screen rather than with the black bars.

I assume this is user error so can anyone tell me what I did wrong on not getting the same output resolution as input and also if there is a standard aspect ratio for Tiktok and other mobile social media video apps?

Do you mean the export job? Check settings → video mode and adjust accordingly.

Also compare the video mode settings to those found in the advanced tab after clicking the export button. Are they the same?