What is up with Hydrus's attention span?

For some time now I've been trying to kill that stupid snake and most of that time has been spent reloading save states on its neck because whenever I start wailing on its last sigil it'll eventually just decide to forget about me, start swimming in the deep and drown me in the procces. It's like every time it puts its head underwater it's a coin flip wether itll continue the fight or do the 'screw yourself' move. Sometimes he does it from disabling the last spike, even. I took a video of one of my attempts - https://www.youtube.com/watch?v=2cya-yk7wKg&feature=youtu.be It is very aggrivating.