All (most?) of UI palettes in Affinity products are top-aligned. I.e., when the palette is larger than the content, the content floats up. But in Export persona, Export options, the content mostly sinks to bottom. (see image with that strange empty space in between). Add to that, when the palette is smaller than the content, a slightly odd positioned scrollbar appears. All this makes me think that this is unintended. And, possibly a bug.. or a papercut.
This issue is present in Designer 2.1 betas, too.
— Theme song: Brian Eno, Long Way Down (album: Another Day on Earth)