I think this is more an issue that the game allows you to turn on quick cast while still using a control scheme like double tap or hold press for the alt cast modes. They might have to have quick cast listen for the key release while the controls are set to those modes.
As for a workaround right now,
I use the alt-cast/self-cast keybind mode to get around this. I can press C to rite an ally or M5+C to rite myself.
Because that works right now, if valve were to change the double tap / hold press modes to listen for key release, they could leave the alt-cast keybind mode's quickcast the way it currently is, listening for keydown, without any loss of functionality.