Aggro

This skill has strange effects with aggro - some monsters tend to only notice you (!) or not notice you at all (no effect) if you use it and back off. Additionally, if you use it in normal stance (not combat) without spam clicking, you remain in normal stance. Has anyone else noticed this? It's kind of like using Flame Burst in Sulfur Spider missions.

Yes, that's just how the skill works in accordance to the mob's AI. Golems, Shadow Creatures, etc. don't care about my existence when I use Kunai Storm, but Spiders have always noticed on non-aggro skills like Stomp or, as you've already mentioned, Flame Burst.