Lissandra Ult not applying "untargetable and invincible"

I had played a game with her not terribly long ago and during a fight in bot, I was being auto'd by a and somehow since she was autoing me before I had ult'd myself. As it took effect, nothing happened besides me being stuck in one spot dealing the self heal/damage. She was stilling hitting me in the process all I did was make myself a immobile target lol.