This happens to me too when I try to cast spells back to back too quickly.

The casting bar will show the name and cast time of the former spell I used, instead of the one I am currently using. So if I cast scourge and then stoneskin quickly after, it will fill up to 100% way before the spell is finished casting, while still displaying the icon for scourge.

Sorry, I don't know how to fix this problem. Just know that you and your friend aren't alone.