tl;dr her ult casts towards your initial cursor position when it was queued, relative to the end of the dash. This is un-intuitive. It should cast in the direction it was queued at. I'm assuming this is also exactly why she has the 'casts towards the target' mechanic on her Q + E.
If there's a champion touching a wall, and you dash towards them and queue R at the same time (basically E + R while hovering the champion), you'll dash into them and throw your ult back in the direction you dashed from, instead of ulting them into the wall as you would expect, because your dash took you slightly ahead of where your cursor was.
And while you're at it, fixing the bug where her Brush Q is sometimes counted as a unit would also be great. It occasionally blocks skill shots or randomly disappears as if it were killed (Yasuo's Q is able to do it often.. Or it might be Shiv).