It's very strange. When you left click on a block while blocking with a sword in creative, on the first swing it does not break a block. It does, however, break a block on the second swing. For example, if you click once for one swing, it just creates particle effects, but if you click again before the animation for the first swing finishes, the block will break. Holding down the mouse button works as normal after the first swing. If you wait until the arm swinging animation finishes, you will again be unable to break the block.