A reel cover is shown in two very different places. In the reel itself and in the Reels tab it is full-screen 9:16. On your profile grid it becomes a 3:4 tile, cut from the centre. Titles placed near the top or bottom of the 9:16 frame disappear from the grid, which is where most people first see your reel.
Design for the 3:4 centre: put the title, the face and the product inside the box drawn on the preview, and use the top and bottom margins for background only.