To further emphasize what d1vio is saying. The stamina cost is correctly 0.5 but for some weird reason you need at least 1 bar of stamina to cast it. I am also having this issue.
To further explain, this stamina requirement only applies at your very last stamina bar. If you are zero stamina, you can't downwards dash when you charge up .5 of a bar, you must wait to acquire 1 bar, but for all bars beyond that, it works properly, also AFAIK still bugged.