Why does TikTok box in my 16:9 videos after posting instead of spanning full width?

Hey everyone, running into an annoying display issue when posting my TikTok video.
In the editor preview screen before posting, the video spans edge-to-edge across the screen. But as soon as I post it, TikTok automatically shrinks the clip into like a box. I attached before and after posting the video. Any idea on how to fix it?