HyperPixel 4" - Video w/ Black Border in full screen

I am running a Video Looper project with pi 3B+ / Raspbain 4.19 / Hyperpixel 4" rectangle display(non-touch), updated and installed the driver of the display. I got the video looper from here: https://learn.adafruit.com/raspberry-pi-video-looper/installation

My issue is: when I make the Looper to play a 800x400 (same as the display) video, it always shows the black bands on the top and the bottom.

I escaped the looper and try to playback a video with VLC player, it only shows the left heft of the picture. However in the full screen mode it works, no black border.

I’ve tried different resolution settings on Raspbian and they all went the same. I do this art project with some different displays and only the Hyperpixel has this issue - although it is a beautiful display. I’ve asked Adafruit’s support but the methods they provided doesn’t work.

Please help me up. I’m a very beginner in this field with zero experience of coding.
Please let me know if there are other ways I could try.