Boss - missing hitbox

Hi,

I was fighting the boss of 'The Titan Grotto', but after he looses a third of his HP, his hitbox disappears. I can not hit the boss with any attack from my mercenary. The ballista does nothing, since it cannot find an enemy hitbox to lock onto.

I tried 3 times and it kept happening, whether I restarted the game or whatever, then I thought of making a clip, and then the problem went away...