So after fiddling with it for a bit, I think this comes down the fact there is no set height on any of the slider’s parent elements, which means even when height:100% is declared it will only ever be as tall as its content.
I would try sticking a height:90vh or similar on the slider somewhere, and maybe switch the container for the slider item/download report button to be grid instead of flex. The slide could be in a row of 90% height and the button in a row of 1fr, or something like that. Best of luck.