Chapter 5 boss- bugged or working as intended?

[Stormcaller]

I'm on the Keeper of the Flaming Mountain boss in Chapter 5 and I can't tell if the fight is bugged or if I'm getting hit with an attack that isn't explained correctly.

After a short while the Destined One stops responding to any controls, not even opening the menu. I know the game is still receiving them though because I can see the game switching from controller UI to Keyboard when I switch back and forth to test. When this happens I can't even die from the bosses' attacks, my health hits zero and stays there.
I wonder if anyone else has experienced this and if this is a bug or if it's a status effect or a unique attack that's very poorly implemented and explained.

[blackc4t]

It's hard to tell what actually happened without screenshot. But during that fight, there are a lot of phases and summons. One of those water attacks makes your character slow