Fiora Blade Waltz's while in Lissandra's Frozen Tomb

Exposition: I am playing Lissandra in a Normal PvP game, going in the Top Lane against enemy champion Fiora. She is able to cast Blade Waltz while encased in Frozen Tomb

Sequence of Events exactly as they happened:

1. Lissandra casts Frozen Tomb on unsuspecting enemy Fiora. Damage is dealt.
2. Fiora casts Blade Waltz after Frozen Tomb animation has fully encased her in a stump of ice.
3. Fiora's Blade Waltz successfully casts and hits Lissandra all 5 times. Fiora's animation while in Blade Waltz is Lissandra's Frozen Tomb ice stump moving around. Damage is dealt.

Issues/Bugs:

1. Fiora is not stunned 1.5 seconds before being allowed to cast Blade Waltz
2. This happened every single time Lissandra cast Frozen Tomb on Fiora (which occurred ~6 times in game)
3. Dancing stump looks hilarious, but sucks when you're dying ;)

Background Information:

1. Fiora had no Tenacity from any source at any point in the game. I checked.
2. Fiora and Lissandra both built standard items.
3. Game was 35 minutes long.
4. The issue/bug occurred in multiple scenarios throughout the game.

Below is end of game build: Lissandra built: 2 Doran's Rings, Death Fire Grasp; Zhonya's Hourglass, Sorcerer's Shoes (no enchantment) and Warding Totem trinket.

Fiora built: Doran's Blade, Hexdrinker, Ravenous Hydra. Youmouu's Ghostblade, Bloodthirster, and Berserker's Greaves (with Homeguard enchantment), and Warding Totem trinket.