If you’re looking for a more descriptive post about the problem, you might want to look here. The issue occurs when the mouse is hovering over any “filled” button on the action bar, the class hall button/icon, and the mini-bar buttons as well. Basically any filled in part of the base UI is breaking mousewheel binds entirely. This breaks the game severely for clickers like myself that use the mouse wheel for things like targeting.
Edit: By “filled” I mean just that. Put a spell/ability bind on an action bar button and leave the button space next to it blank. Assign target next/nearest enemy to the mouse wheel and hover over the action bar button that is filled in with your ability bind. It won’t work. Move the cursor over the “blank” (unfilled) action bar button and it suddenly works. Unit frames are also broken in the same way, preventing mousewheel binds from working.