[Zindrux]

I'm not sure if this information helps. But it seems that the bar and timer shader is either hiding or resetting to zero if I get the error feedback "The spell is not ready yet." If I wait until the cast is over halfway through, I get the "Another action in progress" feedback and the cast bar stays visible.

EDIT: I downloaded Sigg's Konyagi UI to see if the castbars exhibit the same behaviour. They do. To test it, simply start a cast and spam a different spell immediately after the castbar start. You'll see that the castbar for the current spell disappears as you get the "The spell is not ready yet." feedback.