Whirlwind Attack sometimes has no effect at all on monsters in its path when it’s my first attack against the monsters. My asura warrior whirls right through them and nothing else happens on the screen; neither damage nor misses are shown. It also doesn’t seem to matter whether I selected a monster as a target before the attack or not.
In one odd case I had yesterday evening after a series of ineffective Whirlwind Attacks against various monsters, one attack actually did damage but no damage numbers were shown.