Gyobou is immortal apparently?

After like a half hour of attempts, I finally managed to beat him. I finish him, catch my breath, suddenly he's back at 1 HP and broken posture.

No problem, I just finish him again. And again. I'm pretty sure I deathblowed him like 8 times until I just gave up and let him kill me.

Am I missing something here?

Edit: beat him again, still looping (paused this time)

Edit 2: the indicator for finishing mid-deathblow wasn't there until a restart so I'm blaming this on a graphical error